Summary of discussion

[Leonore Reiser]

Leszek
A few comments on some of the points.

> - Consider standardizing the citing of curatorship info. Currently the format is 'GR:pj37'. Consider 'Curator:Gramene,pj37'. Subsequent comment has been that curatorship info. need only be provided where published term definitions are modified by a curator. The latter is a less-tedious route;
I think perhaps I need to explain why some TAIR terms have TAIR:lr
associated to the definitions.
Given that each DB has its own numberspace it should be clear which group
added what term. The reason some TAIR terms have TAIR:lr is because if I
modified the definition then I added myself as a reference for the
definition. For example, if a term came from Esau's Plant Anatomy and I
changed it at all, there are 2 references for the definitions (the ISBN
number
and TAIR:lr). If I made up the definition then the only reference is
TAIR:lr.If the definition went in unchanged then the only reference is the
ISBN # (or other types of references like journal articles).

> 4. Leonore would email a copy of the scanned glossary file from Esau (Anatomy of Seed Plants) - around the group.

I believe I sent to you and Pankaj already- does anyone else want it?
I've attached the text file here. There may still be some typos from
scanning where the OCR wasn't very good. Perhaps one of the programmers
can write a plug in similar to John's to search for terms and import
definitions.

> [Post-meeting comments: what's the intention of the Temporal ontology - generic or taxon-specific or both?? If it is intended to be generic, in the non-taxonomic sense, and taxon-specific, then it would seem that "sensu ..taxon name.." would need to be used for all the taxon-specific info. At present, taxon-specific ontologies are presented].

I just didn't see the point of dividing it along monocot/dicot lines as
higher order nodes.
The nodes I would consider to top the list of plant developmental stages
(including a few I didn't put
in initially -related to the alternation of generations- which I admit is an
appalling error on my part) are the following:

sporophytic(new)
 embryonic
 vegetative
 reproductive
 senesence (new)
gametophytic(which would overlap with sporophytic reproductive stages,
e.g. ovule developmental stages)


> e) noted that the anatomy ontology had some word substitutions - different words associated with the same definition (I think we referred to this as 'word duplications' during the ph.call) - consistency needs to be achieved here;

Mostly it was the same word that was in the ontologies in numerous places
with different IDs. I think hydathode was an example of  a twice used term
(PO:0000323, PO:0005660). Then there were the cases where terms were
given new IDs (e.g. hypocotyl -formerly TAIR/PO:0000310, to PO:0005342, or
petiole, or leaf...).

>
> - the word 'wither' in TAIR-based ontology could be associated with the word 'senesce';

So change the definition of Stage 16 flower to be:
Stage of floral development that begins when the petals and sepals begin
to wither.

???
sure.

> - typos in some of the TAIR-based definitions;

Its a good idea to run spell check (we have ispell for UNIX and
can/should) and fix my
miserable
spelling- I admit to being a very poor typist and bad at spelling.

Regards,
Leonore
-------------------------------------------------------------------------------
Leonore Reiser, Ph.D.                   lreiser at acoma.stanford.edu
The Arabidopsis Information Resource	FAX: (650) 325-6857
Carnegie Institution of Washington	Tel: (650) 325-1521 ext. 311
Department of Plant Biology		URL: http://arabidopsis.org/
260 Panama St.
Stanford, CA 94305
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-------------- next part --------------
Glossary from The Anatomy of Seed Plants 2nd Edition, Katherine Esau (1977) John Wiley and Sons, NY,NY.

ISBN:047125208


Term	Definition

Abaxial.	 Directed away from the axis. Opposite of adaxial.
Abscission.	 The shedding of leaves, flowers, fruits, or other plant parts, usually after formation of an abscission zone.
Abscission layer.	 In abscission zone; layer of cells the disjunction or breakdown of which causes the shedding of a plant part. Other term, separation layer.
Abscission zone.	 Zone at base of leaf, flower, fruit, or other plant part that contains an abscission (or separation) layer and a protective layer, both involved in the abscission of the plant part.
Accessory bud.	 A bud located above or on either side of the main axillary bud.
Accessory cell.	 See subsidiary cell.
Accessory parts in fruit.	 Parts not derived from the ovary but associated with it in fruit.
Accessory transfusion tissue.	 Transfusion tissue located within the mesophyll rather than associated with vascular bundle. In leaves of certain gymnosperms.
Acicular crystal.	 Needle-shaped crystal.
Acropetal development (or differentiation).	 Produced or becoming differentiated in a succession toward the apex of an organ. The opposite of basipetal but means the same as basifugal.
501
Actinomorphic.	 Refers to a flower that can be divided in two equal parts in more than one longitudinal plane; radially symmetrical or regular flower. Opposite of zygomorphic.
Actinostele.	 Protostele with star-shaped outline in transection.
Adaxial.	 Directed toward the axis. Opposite of abaxial.
Adaxial meristem.	 Meristematic tissue on the adaxial side of a young leaf that contributes to the increase in thickness of the petiole and midrib.
Adnation.	 In a flower; union of members of different whorls, as stamens and petals.
Adventitious.	 Refers to structures arising not at their usual sites, as roots originating on stems or leaves instead of on other roots, buds developing on leaves or roots instead of in leaf axils on shoots.
Aerenchyma.	 Parenchyma tissue containing particularly large intercellular spaces of schizogenous, lysigenous, or rhexigenous origin.
Aggregate fruit.	 A fruit developing from a single gynoecium (single flower) composed of separate carpets, as the strawberry or raspberry fruits.
Aggregate ray.		 In secondary vascular tissues; a group of small rays arranged so as to appear to be one large ray.
Albedo.	 White tissue of the rind in citrus fruit.
Albuminous cells.	 In gymnosperm phloem; certain ray and phloem-parenchyma cells spatially and functionally associated with the sieve elements, thus resembling the companion cells of angiosperms but usually not originating from the same precursory cells as the sieve elements. Also called Strasburger cells.
Albuminous seed.	 A seed that contains endosperm in mature state.
Aleurone.	 Granules of protein (aleurone grains) present in seeds, usually restricted to the outermost layer, the aleurone layer of the endosperm. (Protein bodies is the preferred term for aleurone grains.)
Aleurone layer.	 Outermost layer of endosperm in cereals and many other taxa that contains protein bodies and enzymes concerned with endosperm digestion.
Aliform paratracheal parenchyma.	 In secondary xylem; vasicentric groups of axial parenchyma cells having tangential winglike extensions as seen in transections. See also paratracheal parenchyma and vasicentric paratracheal parenchyma.
Alternate pitting.	 In tracheary elements; pits in diagonal rows.
Amoeboid tapetum.	 In anther locules; tapetum assuming amoeboid form when it disintegrates during pollen wall development.
Amphicribral vascular bundle.	 Concentric vascular bundle in which the phloem surrounds the xylem.
Amphiphloic siphonostele.	 A stele in which the vascular system appears as a tube and has phloem both externally and internally from the xylem.
Amphivasal vascular bundle.	 Concentric vascular bundle in which the xylem surrounds the phloem.
Amyloplast.	 A colorless plastid (leucoplast) that forms starch grains.
Analogy.	 Means having the same function as, but a different phylogenetic origin than another entity.
Anastomosis.	 Refers to cells or strands of cells that are interconnected with one another as, for example, the veins in a leaf.
Anatomy.	 The study of structure.
Androecium.	 Collective term for the stamens in a flower of a seed plant; part of the flower in which male gametogenesis is initiated or also carried to completion.
Angiosperm.	 A plant taxon the seed of which is borne within a matured ovary (fruit).
Angstrom (originally Angstrom).	 A unit of length equal to one tenth of a millimicron (mg), or one tenth of a nanometer (nm). Symbol A or A.
Angular collenchyma.	 A form of collenchyma in which the primary wall thickening is most prominent in the angles where several cells are joined.
Anisocytic stoma.	 A stomatal complex in which three subsidiary cells, one distinctly smaller than the other two, surround the stoma.
Anisotropic.	 Having different properties along different axes; optical anisotropy causes polarization and double refraction of light.
Annual ring.	 In secondary xylem: growth ring formed during one season. The term is deprecated because more than one growth ring may be formed during a single year.
Annular cell wall thickening.	 In tracheary elements of the xylem; secondary wal I deposited in the form of rings.
Anomalous secondary growth.	 A term of convenience referring to types of secondary growth that differ from the more familiar ones.
Anomocytic stoma.	 A stoma without subsidiary cells.
Anther.	 The pollen-bearing part of the stamen.
Anthesis.	 The time of full expansion of flower; from development of receptive stigma to fertilization.
Anthocyanin.	 A water-soluble blue, purple, or red flavonoid pigment occurring in the cell sap.
Anticlinal.	 Commonly refers to orientation of cell wall or plane of cell division; perpendicular to the nearest surface. Opposite of periclinal.
Antipodals.	 Three or more cells located at the chalazal end of the mature embryo sac in angiosperms
Aperture in pollen grain.	 A depressed region in the wall in which thick intine is covered by thin exine; the pollen grain emerges through the aperture.
Apex (pl.	 apices), or summit. Tip, topmost part, pointed end of anything. In shoot or root the tip containing the apical meristem.
Apical cell.	 The single cell that occupies the distal position in an apical meristem of root or shoot and is usually interpreted as the initial cell in the apical meristem.
Apical meristem.	 A group of meristematic cells at the apex of root or shoot that by cel I division produce the precursors of the primary tissues of root or shoot: may be vegetative, initiating vegetative tissues and organs, or reproductive, initiating reproductive tissues and organs.
Apocarpy.	 Condition in flower characterized by lack of union of carpels (free carpels).
Apomixis.	 Reproduction by embryo that is formed without meiosis and/or fusion of gametes.
Apotracheal parenchyma.	 In secondary xylem;axial parenchyma typically independent of the vessels (pores). Includes boundary (initial or terminal),banded, and diffuse apotracheal parenchyma.
Apposition.	 Growth of cell wall by successive deposition of wall material, layer upon layer. Opposite of intussusception.
Areole.	 A small area of mesophyll in a leaf delimited by intersecting veins.
Aril.	 A fleshy outgrowth enveloping the seed and usually arising at the base of the ovule.
Articulated laticifer.	 Laticifer composed of more than one cell with common walls intact or partly or entirely removed; anastomosing or nonanastomosing. Compound laticifer.
Aspirated pit.	 In gymnosperm wood; bordered pit in which the pit membrane is laterally displaced and the torus blocks the aperture.
Astrosclereid.	 A branched, or ramified, type of sclereid.
Atactostele.	 A stele with the vascular bundles as though scattered within the ground tissue.
Axial organ.	 Root, stem, inflorescence, or flower axis without its appendages.
Axial parenchyma.	 Parenchyma cells in the axial system of secondary vascular tissues; as contrasted with ray parenchyma cells.
Axial system.	 All secondary vascular cells derived from the fusiform cambial initials and oriented with their longest diameter parallel with the main
Glossary 503
axis of stem or root.	 Other terms: vertical system and longitudinal system.
Axial tracheid.	 Tracheid in the axial system of secondary xylem; as contrasted with ray tracheid.
Axil.	 The upper angle between a stem and a twig or a leaf.
Axillary bud.	 Bud in the axil of a leaf.
Axillary meristem.	 Meristem located in the axil of a leaf and giving rise to an axillary bud.
Banded apotracheal parenchyma.	 In secondary xylem; axial parenchyma in concentric bands as seen in transection, typically independent of vessels (pores). See also apotracheal parenchyma.
Bark.	 A nontechnical term applied to all tissues outside the vascular cambium or the xylem; in older trees may be divided into dead outer bark and living inner bark, which consists of secondary phloem. See also rhytidome.
Bars of Sanio.	 See crassulae.
Basifugal development.	 See acropetal development.
Basipetal development (or differentiation).	 Produced or becoming differentiated in a succession toward the base of an organ. The opposite of acropetal and basifugal.
Bast fiber.	 Originally phloem fiber, now any extraxylary fiber.
Bicollateral vascular bundle.	 A bundle having phloem on two sides of the xylem.
Bifacial leaf.	 A leaf with palisade parenchyma on one side of the blade and spongy parenchyma on the other. A dorsiventral leaf. Conceived ontogenetically, a leaf that develops continuously from the original leaf primordium apex and includes tissues from both adaxial and abaxial sides of the primordium. Compare with unifacial leaf.
Bilateral symmetry.	 Refers to a flower having two corresponding or complementary sides and which thus can be divided, by a single longitudinal plane through the floral axis, in two halves that are mirror images of one another. Contrasted with radial symmetry.
Biseriate ray.	 A ray in secondary vascular tissue two cells in width.
Blind pit.	 A pit without a complementary pit in an adjacent wall, which may face a lumen of a cell or an intercellular space.
Bordered pit.	 A pit in which the secondary wall overarches the pit membrane.
Bordered pit-pair.	 An intercellular pairing of bordered pits.
Boundary apotracheal parenchyma.	 In secondary xylem; axial parenchyma cells occurring either singly or in a layer at the beginning of a season's growth (initial) or at the close of one (terminal). See also apotracheal parenchyma.
Brachysclereid.	 A short, roughly isodiametric sclereid, resembling a parenchyma cell in shape; a stone cell.
Branch gap.	 In the nodal region of a stem; a region of parenchyma in the vascular cylinder of the stem located where the branch traces are bent toward the branch. Usually confluent with the gap of the leaf subtending the branch.
Branch root.	 A root arising from another, older root; also called secondary root if the older root is the primary root, or taproot.
Branch traces.	 Vascular bundles connecting the vascular tissue of the branch and that of the main stem. They are leaf traces of the first leaves (prophylls) on the branch.
Branched pit.	 See ramiform pit.
Bulliform cell.	 An enlarged epidermal cell present, with other similar cells, in longitudinal rows in leaves of grasses. Also called motor cell because of its presumed participation in the mechanism of rolling and unrolling of leaves.
Bundle cap.	 Sclerenchyma or collenchymatous parenchyma appearing like a cap on the xylem and/or phloem side of a vascular bundle as seen in transection.
Bundle sheath.	 Layer or layers of cells enclosing a vascular bundle in a leaf; may consist of parenchyma or sclerenchyma.
Bundle sheath extension.	 A plate of ground tissue extending from a bundle sheath to the epidermis in a leaf; may be present on one or on both sides of the bundle and may consist of parenchyma or sclerenchyma.
Callose.	 A polysaccharide, /3-1,3 glucan, yielding glucose on hydrolysis. Common wall constituent in the sieve areas of sieve elements; also
develops rapidly in reaction to injury in sieve elements and parenchyma cells.	
Callus.	 A tissue composed of large thin-walled cells developing as a result of injury, as in wound healing or grafting, and in tissue culture. (The use of callus for accumulations of callose on sieve areas is deprecated.)
Callus tissue.	 See callus.
Calyptrogen.	 In root apex; meristem giving rise to the rootcap independently of the initials of cortex and central cylinder.
Calyx.	 A collective term for the sepals.
Cambial initials.	 Cells so localized in the vascular cambium or phellogen that their periclinal divisions can contribute cells either to the outside or to the inside of the axis; in vascular cambium, classified into fusiform initials (source of axial cells of xylem and phloem) and ray initials (source of the ray cells).
Cambium.	 A meristem with products of periclinal divisions commonly contributed in two directions and arranged in radial files. Term preferably applied only to the two lateral meristems, the vascular cambium and the cork cambium, or phellogen.
Carpel.	 Leaflike organ in angiosperms producing one or more ovules; a constituent of the gynoecium.
Cartilaginous.	 Like cartilage; a firm elastic material or tissue, translucent in color.
Caruncle.	 A fleshy protuberance near the hilum of a seed.
Casparian strip, or band.	 A bandlike wall formation within primary walls that contains suberin and lignin; typical of endodermal cells in roots, in which it occurs in radial and transverse anticlinalwalls.
Cataphylls.	 Leaves inserted at low levels of plant or shoot, as bud scales, rhizome scales, and others. Contrasted with hypsophylls.
Cauline.	 Belonging to the stem or arising from it.
Caulis.	 Stem.
Cell.	 Structural and physiological unit of a living organism. The plant cell consists of protoplast and cell wall; in nonliving state, of cell wall only, or cell wall and some nonliving inclusions.
Cell membrane.	 A translation of the German Zellmembran which refers to the cell wall in that language.
Cell plate.	 A partition appearing at telophase between the two nuclei formed during mitosis (and some meioses) and indicating the early stage of the division of a cell (cytokinesis) by means of a new cell wall; is formed in the phragmoplast.
Cell wall.	 A more or less rigid membrane enclosing the protoplast of a cell and, in higher plants, composed of cellulose and other organic and inorganic substances.
Cellulose.	 A polysaccharide, /3-1,4 glucan-the main component of cell walls in most plants; consists of long chainlike molecules the basic units of which are anhydrous glucose residues of the formula C,H,o05.
Central cylinder.	 A term of convenience applied to the vascular tissues and associated ground tissue in stem and root. Refers to the same part of stem and root that is designated stele.
Central mother cells.	 Rather large vacuolated cells in subsurface position in apical meristem of shoot in gymnosperms.
Centric mesophyll.	 A modification of isobilateral mesophyll in which the adaxial and abaxial palisade layers form a continuous layer; found in narrow or cylindrical leaves.
Centrifugal development.	 Produced or developing successively farther away from the center.
Centripetal development.	 Produced or developing successively closer to the center.
Chalaza.	 Region in the ovule opposite the micropyle where the integuments and the nucellus merge with the funiculus.
Chimera.	 A plant consisting of a combination of tissues of different genetic composition. In periclinal chimera, cells of different composition are arranged in periclinal layers.
Chlorenchyma.	 Parenchyma tissue containing chloroplasts; leaf mesophyll and other green parenchyma.
Chloroplast.	 A chlorophyll-containing plastid with thylakoids organized into grana and frets, or stroma thylakoids, and embedded in a stroma.
Chromoplast.	 A plastid containing pigments other than chlorophyll, usually yellow and orange carotenoid pigments.
Cicatrice.	 The scar left by a wound or by the separation of one plant part from another (as a leaf
from a stem) and characterized by substances protecting the exposed surface.	
Circular bordered pit.	 A bordered pit with circular aperture.
Cisterna (pl.	 cisternae). A flattened, saclike membranous compartment as in endoplasmic reticulum, dictyosome, or thylakoid.
Cladophyll.	 A branch resembling a foliage leaf.
Closed vascular bundle.	 A bundle forming no vascular cambium.
Closed venation.	 Leaf venation characterized by anastomosing veins.
Closing layer.	 One of the compact layers of cells formed periodically in alternation with the loose filling tissue in a lenticel.
Coenocyte.	 An aggregation of protoplasmic units; a multinucleate structure; sometimes applied to multinucleate cells in seed plants.
Cohesion.	 In a flower; union of members of the same whorl, as sepals with sepals and petals with petals.
Coleoptile.	 The sheath enclosing the epicotyl in the embryo of Poaceae; sometimes interpreted as the first leaf of the epicotyl.
Coleorhiza.	 The sheath enclosing the radicle of the embryo in Poaceae.
Collateral vascular bundle.	 A bundle having phloem only on one side of the xylem usually the abaxial side.
Collenchyma.	 A supporting tissue composed of more or less elongated living cells with unevenly thickened nonlignified primary walls. Common in regions of primary growth in stems and leaves.
Colleter.	 A multicellular appendage or a multicellular trichome producing a sticky secretion. Found on buds of many woody species.
Columella.	 The central part of a rootcap in which the cells are arranged in longitudinal files.
Commissural vascular bundle.	 A small bundle interconnecting larger parallel bundles as in leaves of grasses.
Companion cell.	 A parenchyma cell in the phloem of an agiosperm associated with a sieve tube member and originating jointly with the latter from the same mother cell.
Compitum.	 A region in the style of a syncarpous gynoecium where stylar canals are joined into one cavity and where the pollen tubes can 
change their direction of growth from one carpet to another.	
Complementary tissue.	 See filling tissue.
Complete flower.	 A flower having all types of floral parts: sepals, petals, stamens, and carpets, or tepals, stamens, and carpets.
Compound laticifer.	 See articulated laticifer.
Compound middle lamella.	 A collective term applied to two primary walls and middle lamella; usually used when the true middle lamella is not distinguishable from the primary walls. May also include the earliest secondary wall layers.
Compound sieve plate.	 A sieve plate composed of several sieve areas in either scalariform or reticulate arrangement.
Compression wood.	 The reaction wood in conifers which is formed on the lower sides of branches and leaning or crooked stems and characterized by dense structure, strong lignification, and certain other features.
Concentric vascular bundle.	 A vascular bundle with either the phloem surrounding the xylem (amphicribral ) or the xylem surrounding the phloem (amphivasal ).
Conducting tissue.	 See vascular tissue.
Confluent paratracheal parenchyma.	 In secondary xylem; coalesced aliform groups of axial parenchyma cells forming irregular tangential or diagonal bands, as seen in transection. See also paratracheal parenchyma and aliform paratracheal parenchyma.
Conjunctive tissue.	 A secondary parenchymatic tissue interspersed with vascular tissue where the latter does not form a solid cylinder; as in monocotyledons and in dicotyledons with anomalous secondary growth.
Connate.	 Refers to united parts of the same whorl in a flower, as petals united into a corolla tube. See also cohesion.
Connective.	 The tissue between the two lobes of an anther.
Contact cell.	 An axial parenchyma or a ray cell physiologically associated with a tracheary element. Analogous to companion cell in the phloem. Also a cell next to a stoma.
Contractile root.	 A root that undergoes contraction some time during its development and thereby effects a change in position of the shoot with regard to the ground.
Convergent evolution.	 See parallel evolution.
Coordinated growth.	 Growth of cells in a manner that involves no separation of walls, as opposed to intrusive growth, also called symplastic growth.
Copal.	 A resinous substance exuding from various tropical trees and hardening in air into roundish or irregular pieces, colorless, yellowish, reddish, or brown.
Cork.	 See phellem.
Cork cambium.	 See phellogen.
Cork cell.	 A phellem cell derived from the phellogen, nonliving at maturity, and having subenzed walls; protective in function because the walls are highly impervious to water.
Corolla.	 A collective term for the petals of a flower.
Corolla tube.	 The tubelike part of a corolla resulting from congenital or ontogenetic union of petals.
Corpus.	 The core in an apical meristem covered by the tunica and showing volume growth by divisions of cells in various planes.
Cortex.	 The primary ground tissue region between the vascular system and the epidermis in stem and root. Term also used with reference to peripheral region of a cell protoplast.
Cotyledonary trace.	 The leaf trace of cotyledon located within the hypocotyl and connecting the vascular system of the root with that of the cotyledon.
Crassulae (sing.	 crassula). Thickenings of intercellular material and primary wall along the upper and lower margins of a pit-pair in the tracheids of gymnosperms; also called bars of Sanio.
Cristae (sing.	 crista). Crestlike infoldings of the inner membrane in a mitochondrion:
Cross-field.	 A term of convenience for the rectangle formed by the walls of a ray cell against an axial tracheid; as seen in radial section of the secondary xylem of conifers.
Crystal sand.	 A mass of very fine free crystals.
Crystalloid.	 Protein crystal that is less angular than a mineral crystal and swells in water.
Cuticle.	 A layer of fatty material, cutin, rather impervious to water, located on the outer walls of epidermal cells.
Cuticularization.	 The process of formation of the cuticle.
Cutin.	 A complex fatty substance considerably impervious to water; present in plants as an impregnation of epidermal walls and as a separate layer, the cuticle, on the outer surface of the epidermis.
Cutinization.	 The process of impregnation with cutin.
Cyclosis.	 The streaming of cytoplasm in a cell.
Cystolith.	 A concretion of calcium carbonate on an outgrowth of a cell wall. Occurs in a cell called lithocyst.
Cytochimera.	 A plant with combination of tissues the cells of which have different numbers of chromosomes. See also chimera.
Cyto-histological zonation.	 Presence of regions in the apical meristem having distinctive cytological charateristics. The term is meant to imply that a cytological zonation results in a subdivision into distinguishable tissue regions. Should be replaced with cytological zonation.
Cytokinesis.	 The process of division of a cell as distinguished from the division of the nucleus, or karyokinesis.
Cytological zonation.	 See cyto-histological zonation.
Cytology.	 The science dealing with the cell.
Cytoplasm.	 In a strict definition, the visibly least differentiated part of the protoplasm of a cell that constitutes the groundmass enclosing all other components of the protoplast.Also called hyaloplasm.
Decussate.	 Arrangement of leaves in pairs which alternate with one another at right angles.
Dedifferentiation.	 A reversal in differentiation of a cell or tissue which is assumed to occur when a more or less completely differentiated cell resumes meristematic activity.
Dehiscence.	 The spontaneous opening of a structure, such as anther or fruit, permitting the escape of reproductive entities contained in the dehiscing structure.
Dendroid venation.	 A type of venation in which the minor veins do not form closed meshes about small areas of mesophyll.
Derivative.	 A cell produced by division of a meristematic cell in such a way that it enters the path of differentiation into a body cell; its sister cell may remain in the meristem.
Glossary 507
Dermal tissue.	 See dermal tissue system.
Dermal tissue system.	 The outer covering tissue of a plant, epidermis or periderm.
Dermatogen.	 The meristem forming the epidermis and arising from independent initials in the apical meristem. One of the three histogens, plerome, periblem, and dermatogen, according to Hanstein.
Desmogen.	 Meristematic strand destined to differentiate into a vascular bundle. May be primary, that is, composed of procambial cells, or secondary, that is, derived from a cambium in plants in which the secondary body consists of vascular bundles embedded in secondary parenchyma tissue.
Desmotubule.	 Tubule (often appearing as a solid rod) connecting the two endoplasmic reticulum cisternae located at the two opposite ends of a plasmodesma.
Detached meristem.	 A meristem, with a potenial to give rise to an axillary bud, appearing detached from the apical meristem because of the vacuolation of intervening cells.
Determinate growth.	 Formation of a restricted number of lateral organs by an apical meristem; characteristic of a floral meristem.
Development.	 The change in form and complexity of an organism or part of an organism from its beginning to maturity; combined with growth.
Diacytic stoma.	 A stomatal complex in which one pair of subsidiary cells, with their common walls at right angles to the long axis of the guard cells, surrounds the stoma.
Diaphragms in pith.	 Transverse layers (diaphragms) of firm-walled cells alternating with regions of soft-walled cells which may collapse with age.
Diarch.	 Primary xylem of the root; having two protoxylem strands, or two protoxylem poles.
Dichotomous venation.	 A venation pattern in which the veins appear repeatedly branched into equal portions.
Dictyosome.	 A membrane system (also called organelle) composed of stacked cisternae each producing vesicles at the periphery; when highly active in this process a cisterna may appear netted. Also called golgi body and golgi apparatus.
Dictyostele.	 A stele in which large overlapping leaf gaps dissect the vascular system into anastomosing strands, each with the phloem surrounding the xylem.
Differentiation.	 A physiological and morphological change occurring in a cell, a tissue, an organ, or a plant during development from a meristematic, or juvenile, stage to a mature, or adult, stage. Usually associated with an increase in specialization.
Diffuse apotracheal parenchyma.	 Axial parenchyma in secondary xylem occuring as single cells or as strands distributed irregularly among the fibers, as seen in transection. See also apotracheal parenchyma.
Diffuse porous wood.	 Secondary xylem in which the pores (vessels) are distributed fairly uniformly throughout a growth layer or change in size gradually from early to late wood.
Dilatation.	 Growth of parenchyma by cell division in pith, rays, or axial system in vascular tissues; causes the increase in circumference of bark in stem and root.
Distal.	 Farthest from the point of origin or attachment. Opposite of proximal.
Distichous.	 Arrangement of leaves in two vertical rows; two-ranked arrangement.
Dorsal.	 Equivalent to abaxial in botanical usage.
Dorsiventral leaf.	 Possessing distinct upper and lower sides. Term derived from the reference to the abaxial and adaxial sides of a leaf as dorsal and ventral, respectively. A bifacial leaf.
Double fertilization.	 The fusion of egg and sperm (resulting in a 2n fertilized egg, the zygote) and the simultaneous fusion of the second male gamete with the polar nuclei (resulting in a 3n primary endosperm nucleus). A unique characteristic of angiosperms.
Druse.	 A globular, compound, calcium-oxalate crystal with numerous crystals projecting from its surface.
Duct.	 An elongated space formed by separation of cells from one another (schizogenous origin), by dissolution of cells (lysigenous origin), or by a combination of the two processes (schizo-lysigenous origin); usually concerned with secretion.
Early wood.	 The wood formed in first part of a
growth layer and characterized by a lower density and larger cells than the late wood.	 Term replaces spring wood.
Eccrinous secretion.	 The secretion leaves the cell as individual molecules passing through the plasmalemma and cell wall. Compare with granulocrinous secretion.
Ectodesma.	 See teichode.
Ectophloic siphonostele.	 A stele having a pith and one phloem region, externally to the xylem.
Egg apparatus.	 The egg cell and two synergids located at the micropylar end of the female gametophyte, or the embryo sac, in angiosperms.
Elaioplast.	 A leucoplast type of plastid forming and storing oil.
Elaiosome.	 An outgrowth on a seed or fruit storing oil and serving as food for ants.
Embryo sac.	 The female gametophyte of angiosperms, generally composed of seven cells: the egg cell, two synergids, and three (or more) antipodals (each with a single nucleus), and the central cell (with two nuclei).
Embryogeny (or embryogenesis).	 Formation of embryo.
Embryoid.	 An embryo, often indistinguishable from a normal one, developing not from an egg but from a somatic cell, often in tissue culture.
Enation.	 A term applied to outgrowths of the stem in certain primitive land plants. See also microphyll.
Enation theory.	 A theory that regards microphylls as simple enations in contrast to megaphylls, which are considered to have evolved from branch systems.
Endarch xylem.	 A xylem system in which the maturation of cells progresses centrifugally; that is, the oldest elements (protoxylem) are closest to the center of the axis. Typical of stems in seed plants; also of leaves, in which the protoxylem is on the adaxial side.
Endocarp.	 The innermost layer or layers of the pericarp.
Endodermis.	 The layer of ground tissue forming a sheath around the vascular region and having the casparian strip in its anticlinal walls; may have secondary walls later. It is the innermost layer of the cortex in roots and stems of seed plants.
Endodermoid.	 Resembling the endodermis.
Endogenous.	 Arising from a deep-seated tissue, as a lateral root.
Endomembrane system.	 Collective term for plasmalemma, tonoplast, endoplasmic reticulum, dictyosomes, and the nuclear envelope.
Endoplasmic reticulum.	 (Usually abbreviated to ER.) A system of membranes forming cisternoid or tubular compartments that permeate the cytoplasm. The cisternae appear like paired membranes in sectional profiles. The membranes may be coated with ribosomes (rough or granular ER) or be free of ribosomes (smooth or agranular ER).
Endosperm.	 The nutritive tissue formed within the embryo sac of angiosperms from the central cell containing the primary endosperm nucleus.
Endothecium.	 A wall layer in the anther, usually with secondary wall thickenings.
Endothelium.	 The innermost layer of the integument lining the embryo sac in some taxa. Also called integumentary tapetum.
Enucleate.	 Lacking a nucleus.
Epiblast.	 A small structure opposite the scutellum present in the embryo of some grasses.
Epiblem.	 Term used sometimes for the epidermis of the root. See also rhizodermis.
Epicarp.	 See exocarp.
Epicotyl.	 The shoot part of the embryo or seedling above the cotyledon or cotyledons consisting of an axis and leaf primordia. See also plumule.
Epidermis.	 The outer layer of cells in the plant body primary in origin. If it is multiseriate (multiple epidermis), only the outermost layer differentiates as the epidermis.
Epigeal.	 Said of seed germination when the cotyledon or cotyledons are raised above the surface of the ground. Opposite of hypogeal.
Epigyny.	 Floral structure characterized by the positioning of sepals, petals, and stamens above the ovary (inferior ovary). The tissue enclosing the ovary and bearing the other floral parts above it is interpreted as receptacular tissue in some flowers and as appendicular tissue (floral tube) in others.
Epipetalous stamen.	 A stamen adnate to the corolla.
Epithelium.	 A compact layer of cells, often secre
tory in function, covering a free surface or lining a cavity.	
Epithem.	 Mesophyll of a hydathode concerned with secretion of water.
Ergastic substances.	 Passive products of protoplast such as starch grains, fat globules, crystals, and fluids; occur in cytoplasm, organelles, vacuoles, and cell walls.
Eukaryotic (also eucaryotic).	 Refers to organisms having membrane-bound nuclei, genetic material organized into chromosomes, and membrane-bound cytoplasmic organelles. Opposite of prokaryotic.
Eumeristem.	 Meristem composed of relatively small cells, approximately isodiametric in shape, compactly arranged, and having thin walls, a dense cytoplasm, and large nuclei; word means "true meristem."
Eustele.	 A stele in which the primary vascular tissue is arranged in strands around the pith; typical of gymnosperms and angiosperms.
Exalbuminous seed.	 A seed without endosperm in mature state.
Exarch xylem.	 A xylem system in which the maturation of cells progresses centripetally; that is, the oldest elements (protoxylem) are farthest from the center of the axis. Typical of roots in seed plants.
Exine.	 The outer wall of a spore or a pollen grain.
Exocarp.	 The outermost layer or layers of the pericarp. Epicarp.
Exodermis.	 The outer layer, one or more cells in depth, of the cortex in some roots; a type of hypodermis, the walls of which may be suberized and/or lignified.
Exogenous.	 Arising in superficial tissue, as an axillary bud.
External phloem.	 Primary phloem located externally to the primary xylem.
Extrafloral nectary.	 Nectary occurring on a plant part other than a flower. See also nectary.
Extraxylary fibers.	 Fibers in various tissue regions other than the xylem.
False annual ring.	 One of more than one growth layers formed in the secondary xylem during one growth season, as seen in transection.
Fascicle.	 A bundle. 
Fascicular cambium.	 Vascular cambium originating from procambium within a vascular bundle, or fascicle.
Fascicular tissue system.	 The vascular tissue system.
Fertilization.	 The fusion of two gametes, especially of their nuclei, resulting in the formation of a diploid (2n) zygote.
Festucoid.	 Pertaining to the subfamily of grasses Festucoideae.
Fiber.	 An elongated, usually tapering sclerenchyma cell with a lignified or nonlignified secondary wall; may or may not have a living protoplast at maturity.
Fiber-sclereid.	 A sclerenchyma cell with characteristics intermediate between those of a fiber and a sclereid.
Fiber-tracheid.	 A fiberlike tracheid in the secondary xylem; commonly thick walled, with pointed ends and bordered pits that have lenticular to slitlike apertures.
Fibonacci series.	 Series of numbers formed by successive addition of the last two: 1,.2, 3, 5, 8, 13, 21, 34, etc, These numbers occur in phyllotaxes. The relation was formulated by Leonardo of Pisa, surnamed Fibonacci.
Fibril.	 See macrofibril. Also used to designate long, threadlike subcellular entities in the protop I ast.
Fibrous root system.	 A root system composed of many roots similar in length and thickness; as in grasses and other monocotyledons.
Filament.	 The stalk supporting the anther in a stamen.
File meristem.	 See rib meristem.
Filiform.	 Threadlike.
Filiform apparatus.	 A complex of cell wall invaginations in a synergid cell similar to those in transfer cells.
Filiform sclereid.	 A much elongated slender sclereid resembling a fiber.
Filling tissue.	 The loose tissue formed by the lenticel phellogen toward the outside; may or may not be suberized. Also called complementary tissue.
Flank meristem.	 A misnomer used with reference to the peripheral region of an apical meristem. The use of the word flanks implies that the entity
is two-sided.	 The term should be replaced with peripheral meristem.
Flavedo.	 Yellow-colored tissue of the citrus rind.
Floral apical meristem.	 See apical meristem.
Floral meristem.	 Floral apical meristem. See apical meristem.
Floral nectary.	 See nectary.
Floral tube.	 A tube or cup formed by the united bases of sepals, petals, and stamens, often in perigynous and epigynous flowers.
Florigen.	 A hypothetical hormone assumed to be concerned with the induction of flowering.
Follicle.	 A dry, dehiscent, many-seeded fruit derived from one carpel and splitting along one suture.
Fracture face.	 One of two inner surfaces of a membrane that is exposed when the membrane splits during the fracture process used in freeze etching. There are two fracture faces: the P face (on the protoplasmic half of the membrane), which is closest to the cytoplasm, nucleoplasm, plastid stroma, or mitochondrial matrix; the E face, which is closest to the extracellular, endoplasmic, and exoplasmic space (e.g., vacuole, dictyosome vesicle). (Definition by Branton. D. et al. Science 190:54,1975.)
Free nuclear division.	 Nuclear division (karyokinesis) that occurs without cell wall formation (cytokinesis); characteristic of endosperm in certain taxa.
Fruit wall.	 The outer part of the fruit derived either from the ovary wall (pericarp) or from the ovary wall plus accessory floral parts associated with ovary in fruit.
Fundamental tissue.	 See ground tissue.
Fundamental tissue system.	 See ground tissue system.
Funiculus.	 The stalk of the ovule.
Fusiform cell.	 An elongated cell tapering at the ends.
Fusiform initial.	 In vascular cambium; an elongated cell with approximately wedgeshaped ends that gives rise to the elements of the axial system in the secondary vascular tissues.
Gelatinous fiber.	 A fiber with little or no lignifica-tion and in which part of the secondary wall has a gelatinous appearance.
Genotype.	 The genetic constitution of an organism; contrasted with phenotype.
Geotropism.	 Growth the direction of which is determined by gravity.
Germination.	 The resumption of growth by the embryo in a seed; also beginning of growth of a spore, pollen grain, bud, or other structure.
Gland.	 A multicellular secretory structure.
Glandular hair.	 A trichome having a unicellular or multicellular head composed of secretory cells; usually borne on a stalk of nonglandular cells.
Gluten.	 Amorphous protein occurring in the starchy endosperm of cereals.
Glyoxysome.	 A microbody containing enzymes necessary for converting fats into carbohydrates.
Golgi apparatus.	 A collective term for all golgi bodies, or dictyosomes, in a given cell. Sometimes used for an individual dictyosme.
Golgi body.	 See dictyosome.
Grafting.	 A union of two plants part of one of which, the scion, is inserted on root or stem of the other. the stock
Grana (sing.	 granum). Subunits of chloroplasts seen as green granules with the light microscope and as stacks of disk-shaped cisternae. the thylakoids, with the electron microscope; the chlorophylls and carotenoids associated with photosynthesis are located in the grana.
Granulocrinous secretion.	 The secretion passes an inner cytoplasmic membrane, usually that of a vesicle, and is extruded from the cell after the vesicle fuses with the plasmalemma and releases its contents to the outside. Compare with eccrinous secretion.
Ground meristem.	 A primary meristem, or meristematic tissue, derived from the apical meristem and giving rise to the ground tissues.
Ground tissue.	 Tissues other than the vascular tissues, the epidermis and the periderm; also called fundamental tissue.
Ground tissue system.	 The total complex of ground tissues of the plant.
Growth.	 Increase in size by cell division and/or cell enlargement.
Growth layer.	 A layer of secondary xylem or sec
ondary phloem produced during a single growth period, which may extend through one season (annual ring) or part of one season (false annual ring) if more than one layer is formed in one season; also called growth increment.	
Growth ring.	 A growth layer of secondary xylem or secondary phloem as seen in transection of stem or root; may be an annual ring or a false annual ring.
Guard cells.	 A pair of cells flanking the stomatal pore and causing the opening and closing of the pore by changes in turgor.
Gum.	 A nontechnical term applied to material resulting from breakdown of plant cells, mainly of their carbohydrates.
Gum duct.	 A duct that contains gum.
Gummosis.	 A symptom of a disease characterized by the formation of gum, which may accumulate in cavities or ducts or appear on the surface of the plant.
Guttation.	 Exudation from leaves of water derived from the xylem.
Gynoecium.	 Collective term for the carpels in an angiosperm flower; part of the flower in which female gametogenesis occurs.
Hadrom (or hadrome).	 The tracheary elements and the associated parenchymatic cells of the xylem tissue; the specifically supporting cells (fibers and sclereids) are excluded. (See also leptom.)
Half-bordered pit-pair.	 A pit-pair consisting of a bordered and a simple pit.
Haplocheilic stoma.	 Stomatal type in gymnosperms; subsidiary cells are not related to the guard cells ontogenetically.
Hardwood.	 Technical designation of the wood of dicotyledons.
Haustorium.	 A projection from a cell or tissue that acts as a penetrating and absorbing device. In parasitic angiosperms, a modified root capable of penetrating and absorbing materials from host tissues.
Heartwood.	 The inner layers of secondary xylem that have ceased to function in storage and conduction and in which reserve materials have been removed or converted into heartwood substances; generally darker colored than the functioning sapwood.
Helical cell wall thickening.	 In tracheary elements of the xylem; secondary wall deposited on the primary or secondary wall as a continuous helix; also referred to as spiral cell wall thickening.
Hemicelluloses.	 Polysaccharides more soluble and less ordered than the cellulose; common component of the cell wall matrix.
Heterocellular ray.	 A ray in secondary vascular tissues composed of cells of more than one form: in dicotyledons, of procumbent and square or upright cells; in conifers, of parenchyma cells and ray tracheids.
Heterogeneous ray tissue system.	 Rays in secondary vascular tissues all heterocellular, or homocellular and heterocellular rays are combined. Term not used for conifers.
Hilum.	 (1) The central part of a starch grain around which the layers of starch are arranged concentrically. (2) The scar left by the detached funiculus on a seed.
Histogen.	 Hanstein's term for a meristem in shoot or root tip that forms a definite tissue system in the plant body. Three histogens were recognized: dermatogen, periblem, and plerome. (See the definition of these terms.)
Histogen concept.	 Hanstein's concept stating that the three primary tissue systems in the plant, the epidermis, the cortex, and the vascular system with the associated ground tissue, originate from distinct meristems, the histogens, in the apical meristems. See histogen.
Histogenesis.	 The formation of tissues, hence, histogenetic, having to do with origin or formation of tissues.
Histogenetic.	 See histogenesis.
Homocellular ray.	 A ray in secondary vascular tissues composed of cells of one form only: in dicotyledons, of procumbent, or square, or upright cells; in conifers, of parenchyma cells only.
Homogeneous ray tissue system.	 Rays in secondary vascular tissues all homocellular, composed of procumbent cells only. Term not used for conifers.
Homology.	 Having the same phylogenetic, or evo
lutionary, origin but not necessarily the same structure and/or function.	
Horizontal parenchyma.	 See ray parenchyma.
Horizontal system.	 See ray system.
Hormone.	 A chemical substance produced in one part of an organism and transported to another part in which it has a specific effect.
Hyaloplasm.	 See cytoplasm.
Hydathode.	 A structural modification of vascular and ground tissues, usually in a leaf, that permits the release of water through a pore in the epidermis; may be secretory in function. See epithem.
Hydrolysis.	 The disassembly of large molecules by the addition of water.
Hydromorphic.	 Refers to the structural features of hydrophyfes.
Hydrophyte.	 A plant that requires a large supply of water and may grow partly or entirely submerged in water.
Hygromorphic.	 Synonym of hydromorphic.
Hyperplasia.	 Refers most commonly to an excessive multiplication of cells.
Hypertrophy.	 Refers most commonly to abnormal enlargement. Hypertrophy of a cell or its parts involves no cell division. Hypertrophy of an organ may involve both enlargment of cells and abnormal cell multiplication (hyperplasia).
Hypocotyl.	 Axial part of embryo or seedling located between the cotyledon or cotyledons and the radicle.
Hypocotyl-root axis.	 Axial part of embryo or seedling comprising the hypocotyl and the root meristem, or the radicle if one is present.
Hypodermis.	 A layer or layers of cells beneath the epidermis distinct from the underlying ground tissue cells.
Hypogeal.	 Said of seed germination when the cotyledon or cotyledons remain beneath the surface of the ground. Opposite of epigeal.
Hypogyny.	 Floral structure characterized by the positioning of sepals, petals, and stamens below the ovary (superior ovary).
Hypophysis.	 The uppermost cell of suspensor from which part of the root and rootcap in the embryo of angiosperms are derived.
Hypsopylls.	 Leaves inserted at high levels of a plant, as floral bracts. Contrasted with cataphylls.
Idioblast.	 A cell in a tissue that markedly differs in form, size, or contents from other cells in the same tissue.
Imperfect flower.	 A flower lacking either stamens or carpels.
Included phloem.	 Secondary phloem included in the secondary xylem of certain dicotyledons. Term replaces interxylary phloem.
Increment.	 In growth, an addition to the plant body by the activity of a meristem.
Indehiscent.	 Not opening spontaneously; refers to a certain fruit type.
Indeterminate growth.	 Refers to apical meristem producing an unrestricted number of lateral organs; characteristic of vegetative apical meristem.
Inferior ovary.	 See epigyny.
Initial.	 (1) Cell in a meristem that by division gives rise to two cells one of which remains in the meristem, the other is added to the plant body. (2) Sometimes used to designate a cell in its earliest stage of specialization. More appropriate term for (2), primordium.
Initial apotracheal parenchyma.	 See boundary apotracheal parenchyma.
Inner bark.	 See bark.
Integument.	 Outer cell layers enveloping the nucellus of the angiosperm ovule and differentiating into the seed coat.
Integumentary tapetum.	 The deeply staining innermost integumentary epidermis lining the embryo sac in some taxa and apparently assisting in the nutrition of the embryo. Also called endothelium.
Intercalary growth.	 Growth by cell division that occurs some distance from the meristem in which the cells originated.
Intercalary meristem.	 Meristematic tissue derived from the apical meristem and continuing meristematic activity some distance from that meristem; may be intercalated between tissues that are no longer meristematic.
Intercellular space.	 A space between two or more
cells in a tissue; may have schizogenous, ly-
sigenous, schizo-lysigenous, or rhexigenous origin.	
Intercellular substance.	 See middle lamella.
Interfascicular cambium.	 Vascular cambium arising between vascular bundles (fascicles) in the interfascicular parenchyma.
Interfascicular region.	 Tissue region located between the vascular bundles (fascicles) in a stem; also called medullary or pith ray.
Intermediary cell.	 A parenchyma cell in the phloem of a minor vein that serves as a connection between the photosynthetic tissue and the sieve tube system; may be a companion cell. May or may not have wall ingrowths. See transfer cell.
Internal phloem.	 The primary phloem located internally from the primary xylem. Term replaces intraxylary phloem.
Internode.	 The region between nodes of a stem.
Interpositional growth.	 See intrusive growth.
Intervascular pitting.	 The pitting between tracheary elements.
Interxylary cork.	 The cork that develops within the xylem tissue.
Interxylary phloem.	 See included phloem.
Intine.	 The inner wall layer of a pollen grain or spore.
Intraxylary phloem.	 See internal phloem.
Intrusive growth.	 A type of growth in which a growing cell intrudes between other cells that separate from each other along the middle lamella in front of the tip of the growing cell; also called interpositional growth.
Intussusception.	 Growth of cell wall by interpolation of new wall material within previously formed wall. Opposite of apposition.
Irregular flower.	 A flower in which one or more members of at least one whorl of the perianth differ in form from other members of the same whorl and which cannot be divided in two equal halves in more than one plane. See bilateral symmetry.
Isobilateral leaf.	 A leaf in which the palisade parenchyma occurs on both sides of the leaf. See also unifacial leaf.
Isobilateral mesophyll.	 See isobilateral leaf.
plant, as floral bracts.	 Contrasted with cataphylls.
Idioblast.	 A cell in a tissue that markedly differs in form, size, or contents from other cells in the same tissue.
Imperfect flower.	 A flower lacking either stamens or carpels.
Included phloem.	 Secondary phloem included in the secondary xylem of certain dicotyledons, Term replaces interxylary phloem.
Increment.	 In growth, an addition to the plant body by the activity of a meristem.
Indehiscent.	 Not opening spontaneously; refers to a certain fruit type.
Indeterminate growth.	 Refers to apical meristem producing an unrestricted number of lateral organs; characteristic of vegetative apical meristem.
Inferior ovary.	 See epigyny.
Initial.	 (1) Cell in a meristem that by division gives rise to two cells one of which remains in the meristem, the other is added to the plant body. (2) Sometimes used to designate a cell in its earliest stage of specialization. More appropriate term for (2), primordium.
Initial apotracheal parenchyma.	 See boundary apotracheal parenchyma.
Inner bark.	 See bark.
Integument.	 Outer cell layers enveloping the nucellus of the angiosperm ovule and differentiating into the seed coat.
Integumentary tapetum.	 The deeply staining innermost integumentary epidermis lining the embryo sac in some taxa and apparently assisting in the nutrition of the embryo. Also called endothelium.
Intercalary growth.	 Growth by cell division that occurs some distance from the meristem in which the cells originated.
Intercalary meristem.	 Meristematic tissue derived from the apical meristem and continuing meristematic activity some distance from that meristem; may be intercalated between tissues that are no longer meristematic.
Intercellular space.	 A space between two or more
cells in a tissue; may have schizogenous, lysigenous, schizo-lysigenous, or rhexigenous origin.	
Intercellular substance.	 See middle lamella.
Interfascicular cambium.	 Vascular cambium arising between vascular bundles (fascicles) in the interfascicular parenchyma.
Interfascicular region.	 Tissue region located between the vascular bundles (fascicles) in a stem; also called medullary or pith ray.
Intermediary cell.	 A parenchyma cell in the phloem of a minor vein that serves as a connection between the photosynthetic tissue and the sieve tube system; may be a companion cell. May or may not have wall ingrowths. See transfer cell.
Internal phloem.	 The primary phloem located internally from the primary xylem. Term replaces intraxylary phloem.
Internode.	 The region between nodes of a stem.
Interpositional growth.	 See intrusive growth.
Intervascular pitting.	 The pitting between tracheary elements.
Interxylary cork.	 The cork that develops within the xylem tissue.
Interxylary phloem.	 See included phloem.
Intine.	 The inner wall layer of a pollen grain or spore.
Intraxylary phloem.	 See internal phloem.
Intrusive growth.	 A type of growth in which a growing cell intrudes between other cells that separate from each other along the middle lamella in front of the tip of the growing cell; also called interpositional growth.
Intussusception.	 Growth of cell wall by interpolation of new wall material within previously formed wall. Opposite of apposition.
Irregular flower.	 A flower in which one or more members of at least one whorl of the perianth differ in form from other members of the same whorl and which cannot be divided in two equal halves in more than one plane. See bilateral symmetry.
Isobilateral leaf.	 A leaf in which the palisade parenchyma occurs on both sides of the leaf. See also unifacial leaf.
Isobilateral mesophyll.	 See isobilateral leaf.
Isodiametric.	 Regular in form, with all diameters equally long.
Isolateral leaf.	 Synonym of isobilateral leaf.
Isotropic.	 Having the same properties along all axes. Optically isotropic material does not affect the light.
Karyokinesis.	 Division of a nucleus as distinguished from the division of the cell, or cytokinesis
Lacuna (pl.	 lacunae). Space. Usually air space between cells, which may be schizogenous, lysigenous, schizo-lysigenous or rhexigenous in origin. Also used with reference to the leaf gap.
Lacunar collenchyma.	 A collenchyma characterized by intercellular spaces and cell wall thickenings facing the spaces.
Lamella.	 A thin plate or layer.
Lamellar collenchyma.	 A collenchyma in which cell wall thickenings are deposited mainly on tangential walls.
Lamina of leaf.	 Blade, or expanded part of the leaf.
Late wood.	 The secondary xylem formed in the later part of a growth layer; denser and composed of smaller cells than the early wood. Term replaces summer wood.
Lateral meristem.	 A meristem located parallel with the sides of the axis; refers to the vascular cambium and phellogen, or cork cambium.
Latex (pl, latices).	 A fluid, often milky, contained in laticifers; consists of a variety of organic and inorganic substances, often including rubber.
Laticifer.	 A cell or a cell series containing a characteristic fluid called latex.
Laticiferous cell.	 A nonarticulated, or simple, laticifer.
Laticiferous vessel.	 An articulated, or compound, laticifer in which the cell walls between contiguous cells are partly or completely removed.
Leaf buttress.	 A lateral protrusion below the apical meristem constituting the initial stage in the development of a leaf primordium.
Leaf fibers.	 Technical designation of fibers derived from monocotyledons, chiefly from their leaves.
Leaf gap.	 A region of parenchyma in the vascular
cylinder of a stem located above the level where a leaf trace diverges toward the leaf.	 Also called lacuna. Involves no interruption of vascular connections.
Leaf sheath.	 The lower part of a leaf that invests the stem more or less completely.
Leaf trace.	 A vascular bundle in the stem extending between its connection with a leaf and that with another vascular unit in the stem; a leaf may have one or more leaf traces.
Lenticel.	 An isolated region in the penderm distinguished from the phellem in having intercellular spaces; the tissue may or may not be suberized.
Leptom (or leptome).	 The sieve elements and the associated parenchymatic cells of the phloem tissue; the supporting cells (fibers and sclereids) are excluded. (See also hadrom )
Leucoplast.	 A colorless plastid.
Libriform fiber.	 A xylem fiber commonly with thick walls and simple pits: usually the longest cell in the tissue.
Light line.	 A continuous line parallel with the surface seen in sections of the epidermis of certain leguminous seeds; results from matching of strongly refractive regions in the cell walls of contiguous epidermal cells.
Lignification.	 The impregnation with lignin.
Lignin.	 An organic substance or mixture of substances of high carbon content derived from phenylpropane and distinct from carbohydrates. Associated with cellulose in the walls of many cells.
Lithocyst.	 A cell containing a cystolith.
Locule.	 The cavity within a sporangium containing the spores (e.g., pollen grains) or within an ovary containing the ovules.
Longitudinal parenchyma.	 See axial parenchyma.
Longitudinal system.	 In secondary vascular tissues. See axial system.
Lumen of a cell.	 The space bound by the cell wall.
Lutoids.	 Vesicles, also called vacuoles, in laticifers bound by a single membrane and containing a spectrum of hydrolytic enzymes capable of degrading most of the organic compounds in the cell.
Lysigenous.	 As applied to an intercellular space, originating by a dissolution of cells.
Lysis.	 A process of disintegration or degradation.
Lysosomal compartment.	 A region in the cell protoplast or cell wall where acid hydrolases, capable of digesting cytoplasmic constituents and metabolites, are localized. Bound by a single membrane in the protoplast and usually constituting the vacuolar system. Another term, lytic compartment.
Lysosome.	 An organelle bound by a single membrane and containing acid hydrolytic enzymes capable of breaking down proteins and other organic macromolecules; in plants, represented by vacuoles. See lysosomal compartment.
Lytic compartment.	 See lysosomal compartment.
Maceration.	 The artificial separation of cells of a tissue by causing a disintegration of the middle lamella.
Macrofibril.	 An aggregation of microfibrils in a cell wall visible with the light microscope.
Macrosclereid.	 Elongated sclereid with unevenly distributed secondary wall thickenings; common in seed epidermis of Fabaceae.
Malpighian cell.	 Synonym of macrosclereid.
Macrospore.	 See megaspore.
Mantle.	 The outer layers of the kind of apical meristem that shows a layered arrangement of cells.
Marginal growth.	 The growth along the margins of a leaf primordium that results in the formation of the blade.
Marginal initials.	 Cells along the margins of a growing leaf lamina that contribute cells to the protoderm.
Marginal meristem.	 The meristem along the margin of a leaf primordium concerned with marginal growth of the blade.
Mass meristem.	 A meristematic tissue in which the cells divide in various planes so that the tissue increases in volume.
Matrix.	 Generally refers to a medium in which something is embedded.
Mechanical tissue.	 See supporting tissue.
Medulla.	 Synonym for pith.
Medullary bundles.	 Vascular bundles located in the pith region.
Medullary ray.	 See interfascicular region.
Medullary sheath.	 See perimedullary region.
Megagametophyte.	 Female gametophyte in het
erosporous plants; embryo sac within the ovule in angiosperms.	
Megaphyll.	 Leaf in ferns and seed plants larger than the microphyll in lower taxa and having a leaf trace associated with a leaf gap._
Megasporangium.	 Sporangium in which megaspores are produced; nucellus of the ovule in angiosperms.
Megaspore.	 A haploid (in) spore developing into a female gametophyte in heterosporous plants.
Megaspore mother cell.	 See megasporocyte.
Megasporocyte.	 A diploid (2n) cell that undergoes meiosis and produces four haploid (1n) megaspores; also called megaspore mother cell.
Megasporophyll.	 A leaflike organ producing meagsporangia; carpel producing ovules in the angiosperms.
Meristele.	 One  of the vascular bundles of a dictyostele.
Meristem Tissue primarily concerned with protoplasmic synthesis and formation of new cells by division.	
Meristematic cell.	 A cell synthesizing protoplasm and producing new cells by division; varies in form, size, wall thickness, and degree of vacuolation, but has only a primary cell wall.
Meristemoid.	 A cell or a group of cells constituting an active locus of meristematic activity in a tissue composed of somewhat older, differentiating cells.
Mesarch xylem.	 A xylem strand in which the protoxylem is in the center and the metaxylem differentiates both centripetally and centrifugally from the center.
Mesocarp.	 The middle layer of a pericarp.
Mesocotyl.	 The internode between the scutellar node and the coleoptile in the embryo and seedling of Poaceae.
Mesomorphic.	 Refers to structural features of mesophytes.
Mesophyll.	 The photosynthetic parenchyma of a leaf blade located between the two epidermal layers.
Mesophyte.	 A plant requiring an environment that is neither too wet nor too dry.
Mestome sheath.	 An endodermoid sheath of a vascular bundle; the inner of two sheaths in leaves of Poaceae, mainly those of the festucoid subfamily.
Metacutisation.	 Depostion of suberin lamellae in outer cells of root tips that cease to be active in growth and absorption at the end of seasonal growth. Late suberization.
Metaphloem.	 Part of the primary phloem that differentiates after the protophloem and before the secondary phloem, if any of the latter is formed in a given taxon.
Metaxylem.	 Part of the primary xylem that differentiates after the protoxylem and before the secondary xylem, if any of the latter is formed in a given taxon.
Micelles.	 The regions in cellulose microfibrils in which the cellulose molecules are arranged parallel to each other so that a crystalline lattice structure is present.
Microbody.	 Organelle bound by a single membrane and containing various enzymes except the hydrolytic. Peroxisomes and glyoxysomes are microbodies.
Microfibril.	 A threadlike component of the cell wall consisting of cellulose molecules and visible only with the electron microscope.
Microgametophyte.	 The male gametophyte of a heterosporous plant; pollen grain in a seed p I ant.
Micrometer.	 One thousandth millimeter; also called micron. Symbol wm.
Micromicron.	 One millionth micron. One hundredth angstrom. Symbol
Micron.	 See micrometer.
Microphyll.	 Small leaf the trace of which is not associated with a leaf gap. Contrasted with megaphyll. See also enation.
Micropyle.	 The opening in the integuments of an ovule through which the pollen tube usually enters the embryo sac.
Microsporangium.	 Sporangium in which microspores are formed; anther locule and its walls in angiosperms.
Microspore.	 A haploid (1n) spore developing into a male gametaphyte in heterosporous plants; the uninucleate pollen grain in seed plants.
Microspore mother cell.	 See microsporocyte.
Microsporocyte.	 A diploid (2n) cell that undergoes meiosis and forms four haploid (1n) mi
crospores; also called microspore mother cell and, in seed plants, pollen mother cell.	
Microsporophyll.	 Leaflike organ bearing microsporangia; in angiosperms, modified into stamen.
Microtubules.	 Nonmembranous tubules about 25 manometers (250 angstroms) in diameter and of indefinite length. Located in the cytoplasm in a nondividing eukaryotic cell, usually rear the cell wall, and form the meiotic or mitotic spindle and the phragmoplast in a dividing cell. Sometimes called organelles.
Middle lamella.	 Layer of intercellular material, chiefly pectic substances, cementing together the primary walls of contiguous cells.
Millimicron.	 See manometer.
Mitochondrion (pl, mitochondria).	 Double membrane-bound cell organelle concerned with respiration, carries enzymes and is the major source of ATP in nonphotosynthetic cells.
Morphogenesis.	 Development of form; the sum of phenomena of development and differentiation of tissues and organs.
Morphology.	 The study of form and its development.
Mother cell.	 See precursory cell.
Motor cell.	 See bulliform cell.
Mucilage cell.	 Cell containing mucilages or gums or similar carbohydrate material characterized by the property of swelling in water.
Mucilage duct.	 A duct containg mucilage or gum or similar carbohydrate material. See also duct.
Multilacunar node.	 A node in stem having numerous gaps (and numerous leaf traces) related to one leaf.
Multiperforate perforation plate.	 In vessel member of the xylem; a perforation plate that has more than one perforation.
Multiple epidermis.	 A tissue two or more cell layers deep derived from the protoderm; only the outermost layer differentiates as a typical epidermis.
Multiple fruit.	 A fruit composed of several matured ovaries each produced in a separate flower.
Multiseriate ray.	 A ray in secondary vascular tissues that is few to many cells wide.
Mycorrhiza (pl.	 mycorrhizae). The symbiotic association of fungi and roots of higher plants. May be ectotrophic (web of hyphae invests the root of the host) or endotrophic (the hypae are located within the root cells).
Myrosin cell.	 Cell containing glucosinolates ("mustard oil glucosides") and myrosinases, enzymes hydrolysing the glucosinolates. Occur in eleven dicotyledon families, the two largest of which are the Brassicaceae and Euphorbiaceae.
Nacre wall.	 See nacreous wall.
Nacreous wall.	 A nonlignified wall thickening that is often found in sieve elements and resembles a secondary wall when it attains considerable thickness; designation based on glistening appearance of the wall.
Nanometer.	 One millionth of a millimeter; symbol nm. Equal to one millimicron (mg) or 10 angstroms (A).
Nectary.	 A multicellular glandular structure secreting a liquid containing organic substances including sugar. Occurs in flowers (floral nectary) and on vegetative plant parts (extrafloral nectary).
Netted venation.	 See reticulate venation.
Nodal diaphragm.	 A septum of tissue at the node of a stem extending across the otherwise hollow pith region.
Node.	 That part of the stem at which one or more leaves are attached; not sharply delimited anatomically.
Nodular end wall.	 The cell wall, at right angles to the longitudinal axis of a xylem parenchyma cell, that appears beaded because of deeply depressed pits.
Nodules.	 Enlargements on roots of plants,particularly in the Fabaceae, inhabited by nitrogenfixing bacteria.
Nonarticulated laticifer.	 A simple laticifer consisting of a single, commonly multinucleate, cell; may be branched or unbranched.
Nonporous wood.	 Secondary xylem having no vessels.
Nonstoried cambium.	 Vascular cambium in which the fusiform initials and rays are not arranged in horizontal tiers on tangential surfaces. Nonstratitied cambium.
Nonstoried wood.	 Secondary xylem in which the axial cells and rays are not arranged in horizon
tal tiers on tangential surfaces.	 Nonstratified wood.
Nonstratified cambium.	 See nonstoried cambium.
Nonstratified wood.	 See nonstoried wood.
Nucellus.	 Inner part of an ovule in which the embryo sac develops. Commonly considered to be equivalent to the megasporangium.
Nuclear envelope.	 The double membrane enclosing the nucleus of a cell.
Nucleolar organizer.	 A region on a certain chromosome concerned with the formation of nucleolus.
Nucleolus.	 Spherical body, composed mainly of RNA and protein, present in the nucleus of eukaryotic cells, one or more to a nucleus; site of synthesis of ribosomes.
Nucleus.	 In biology, organelle in a eukaryotic cell bound by a double membrane and containing the chromosomes, nucleoli, and nucleoplasm.

Obturator.	 An outgrowth of the placenta or of the lining of stylar canal that brings the pollen transmitting tissue close to the micropyle.
Ontogeny.	 The development of an organism, organ, tissue, or cell from inception to maturity.
Open vascular bundle.	 A bundle in which the procambium differentiates into vascular cambium after all primary vascular cells are formed.
Open venation.	 Leaf venation in which large veins end freely in the mesophyll instead of being connected by anastomoses with other veins.
Opposite pitting.	 Pits in tracheary elements disposed in horizontal pairs or in short horizontal rows.
Organ.	 A distinct and visibly differentiated part of a plant, such as root, stem, leaf, or part of a flower.
Organelle.	 A distinct body within the cytoplasm of a cell, specialized in function.
Orthostichy.	 A vertical line along which is attached a series of leaves or scales on an axis of a shoot or shootlike organ. Often incorrectly applied to a steep helix, or parastichy.
Osteosclereid.	 Bone-shaped sclereid having a columnar middle part and enlargements at both ends.
Outer bark.	 See bark.
Ovary.	 Lower part of a carpel (simple pistil) or of a gynoecium composed of united carpets (compound pistil) containing the ovules and differentiating into the fruit.
Ovule.	 A structure in seed plants enclosing the female gametophyte and composed of the nucellus, one or two integuments, and funiculus; differentiates into the seed.
Paedomorphosis.	 Delay in evolutionary advance in some characteristics as compared with others resulting in a combination of juvenile and advanced characteristics in the same cell, tissue, or organ.
Palisade parenchyma.	 Leaf mesophyll parenchyma characterized by elongated form of cells and their arrangement with their long axes perpendicular to the surface of the leaf.
Panicoid.	 Pertaining to the subfamily of grasses Pan i coi deae.
Papilla (pl.	 papillae). A soft protuberance on an epidermal cell; a type of trichome.
Paracytic stoma.	 A stomatal complex in which one or more subsidiary cells flank the stoma parallel with the long axes of the guard cells.
Paradermal.	 Parallel with the epidermis. Refers specifically to a section made parallel with the surface of a flat organ such as a leaf; it is also a tangential section.
Parallel evolution.	 Evolution occurring in a similar direction in different taxa. Also called convergent evolution.
Parallel venation.	 Main veins in a leaf blade arranged approximately parallel to one another, although converging at base and apex of leaf.
Parastichy.	 A helix along which is attached a series of leaves or scales on an axis of a shoot or shootlike organ. See also orthostichy.
Paratracheal parenchyma.	 Axial parenchyma in secondary xylem associated with vessels and other tracheary elements. Includes aliform, confluent, and vasicentric.
Parenchyma.	 Tissue composed of parenchyma cells.
Parenchyma cell.	 Typically a not distinctly specialized cell with a nucleate protoplast concerned with one or more of the various physio
logical and biochemical activities in plants.	 Varies in size, form, and wall structure.
Parietal cytoplasm.	 Cytoplasm located next to the cell wall.
Parthenocarpy.	 The development of fruit without fertilization. The fruit is usually seedless.
Passage cell.	 Cell in exodermis or endodermis that remains thin walled when the associated cells develop thick secondary walls. Has casparian strip in endodermis.
Pectic substances.	 A group of complex carbohydrates, derivatives of polygalacturonic acid, occurring in plant cell walls; particularly abundant as a constituent of the middle lamella.
Pedicel.	 The stalk of an individual flower.
Peduncle.	 The stem of an inflorenscence.
Peltate hair.	 A trichome consisting of a discoid plate of cells borne on a stalk or attached directly to the basal foot cell.
Perfect flower.	 A flower having both carpets and stamens.
Perforation plate.	 Part of a wall of a vessel member that )s perforated.
Perianth.	 Petals and sepals or tepals of a flower considered together.
Periblem.	 The meristem forming the cortex. One of the three histogens, plerome, periblem, and dermatogen, according to Hanstein.
Pericarp.	 Fruit wall developed from the ovary wall.
Periclinal.	 Commonly refers to orientation of cell wall or plane of cell division; parallel with the circumference or the nearest surface of an organ. Opposite of anticlinal. See also tangential.
Periclinal chimera.	 See chimera.
Pericycle.	 Part of ground tissue of the stele located between the phloem and the endodermis. In seed plants, regularly present in roots, absent in most stems.
Pericyclic fiber.	 See perivascular fiber.
Pericyclic sclerenchyma.	 See perivascular
sclerenchyma.	
Periderm.	 Secondary protective tissue that replaces the epidermis in stems and roots, rarely in other organs. Consists of phellem (cork), phellogen (cork cambium), and phelloderm.
Perigyny.	 Floral structure characterized by an extension above the receptacle resembling a cup and bearing the sepals, petals, and stamens. The cup may be receptacular or appendicular (floral tube) in origin.
Perimedullary region or zone.	 The peripheral region of the pith (medulla); also called medullary sheath.
Perinuclear space.	 Space between the two membranes forming the nuclear envelope.
Perisperm.	 Storage tissue in seed similar to endosperm but derived from the nucellus.
Perivascular fiber.	 A fiber located along the outer periphery of the vascular cylinder in the axis of a seed plant and not originating in the phloem. Alternate term, pericyclic fiber.
Perivascular sclerenchyma.	 Sclerenchyma located along the outer periphery of the vascular cylinder and not originating in the phloem. Alternate term, pericyclic sclerenchyma.
Peroxisome.	 A cell organelle of the type of microbody that is involved in glycolic acid metabolism associated with photosynthesis.
Petal.	 A unit of the corolla of a flower.
Petiole.	 Stalk of a leaf.
Phellem (cork).	 Protective tissue composed of nonliving cells with suberized walls and formed centrifugally by the phellogen (cork cambium) as part of the periderm. Replaces the epidermis in older stems and roots of many seed plants.
Phelloderm.	 A tissue resembling cortical parenchyma produced centripetally by the phellogen (cork cambium) as part of the periderm of stems and roots in seed plants.
Phellogen (cork cambium).	 A lateral meristem forming the periderm, a secondary protective tissue common in stems and roots of seed plants. Produces phellem (cork) centrifugally, phelloderm centripetally by periclinal divisions.
Phelloid cell.	 A cell within the phellem (cork) but distinct from the cork cell in having no suberin in its walls. May be a sclereid.
Phenotype.	 Physical appearance of an organism resulting from interaction between its genotype (genetic constitution) and the environment.
Phlobaphenes.	 Anhydrous derivatives of tannins. Amorphous yellow, red, or brown substances very conspicuous when present in cells.
Phloem.	 The principal food-conducting tissue of the vascular plant composed mainly of sieve elements, various kinds of parenchyma cells, fibers, and sclereids.
Phloem elements.	 Cells of the phloem tissue.
Phloem initial.	 A cambial cell on the phloem side of the cambial zone that is the source of one or more cells arising by periclinal divisions and differentiating into phloem elements with or without additional divisions in various planes. Sometimes called phloem mother cell.
Phloem mother cell.	 A cambial derivative that is the source of certain elements of the phloem tissue, such as, sieve element and its companion cells or phloem parenchyma cells forming a parenchyma strand. Used also in a wider sense synonymously with phloem initial.
Phloem parenchyma.	 Parenchyma cells located in the phloem. In secondary phloem refers to axial parenchyma.
Phloem ray.	 That part of a vascular ray which is located in the secondary phloem.
Phloic procambium.	 That part of procambium which differentiates into primary phloem.
Photoperiodism.	 Response to duration and timing of day and night expressed in the character of growth, development, and flowering in plants.
Photorespiration.	 The light-dependent production of glycolic acid in chloroplasts and its subsequent oxidation in peroxisomes.
Photosynthetic cell.	 A chloroplast-containing cell engaged in photosynthesis.
Phragmoplast.	 Fibrous structure (light microscope view) that arises between the daughter nuclei at telophase and within which the initial partition (cell plate), dividing the mother cell in two (cytokinesis), is formed. Appears at first as a spindle connected to the two nuclei, but later spreads laterally in the form of a ring. Consists of microtubules.
Phragmosome.	 Layer of cytoplasm formed across the cell where the nucleus becomes located and divides. The equatorial plane of the subsequently appearing phragmoplast coincides with the plane of the cytoplasmic layer. In electron microscopy, the term refers to enzyme-containing microbodies that are thought to be participating in cell plate formation.
Phyllode.	 Supposedly a flat, expanded petiole replacing the leaf blade in photosynthetic function.
Phyllotaxy (or phyllotaxis).	 The mode in which the leaves are arranged on the axis of a shoot.
Phylogeny.	 Evolutionary history of a species or larger taxon.
Phytochrome.	 A proteinaceous pigment occurring in the cytoplasm of green plants and serving as a photoreceptor for red-far-red light. Involved in timing certain processes such as dormancy, leaf formation, flowering, and seed germination.
Pinocytosis.	 Process of uptake of a substance by the protoplast through invaginations of the plasmalemma  that are pinched off as vesicles (pinocytotic vesicles) the contents of which are incorporated into the cytoplasm.
Pinocytotic vesicles.	 See pinocytosis.
Pistil.	 Equivalent to the whole syncarpous gynoecium (compound pistil) or to a single carpel in an apocarpous gynoecium (simple pistil). Is composed of ovary, style, and stigma.
Pistillate.	 Refers to a flower with one or more carpels but no functional stamens.
Pit.	 A recess or cavity in the cell wall where the primary wall is not covered by secondary wall. Pitlike structures in the primary wall are designated primordial pits, primary pits, or primary pitfields. A pit is usually a member of a pit-pair.
Pit aperture.	 Opening into the pit from the interior of the cell. If a pit canal is present in a bordered pit, two apertures are recognized, the inner, from the cell lumen into the canal, and the outer, from the canal into the pit cavity.
Pit canal.	 The passage from the cell lumen to the chamber of a bordered pit. (Simple pits in thick walls usually have canallike cavities.)
Pit cavity.	 The entire space within a pit from pit membrane to the cell lumen or to the outer pit aperture if a pit canal is present.
Pit-field.	 See primary pit-field.
Pit membrane.	 The part of the intercellular layer and primary cell wall that limits a pit cavity externally.
Pit-pair.	 Two complementary pits of two adjacent
cells.	 Essential components are two pit cavities and the pit membrane.
Pith.	 Ground tissue in the center of a stem or root. Homology of pith in root and stem is uncertain.
Pith ray.	 See interfascicular region.
Placenta (pl.	 placentae). Region in the ovary where ovules originate and remain attached to maturity.
Placentation.	 Refers to the distribution of placentae in an ovary.
Plasma membrane.	 See plasmalemma.
Plasmalemma.	 Single membrane delimiting the cytoplasm next to the cell wall. A type of unit membrane. Also called plasma membrane.
Plasmodesma (pl.	 plasmodesmata. A connection of protoplasts of two contiguous cells through a canal in the cell wall. Consists of plasmalemma lining the canal, a central desmotubule connecting two endoplasmic reticulum cisternae located at the two opposite openings of the canal, and cytoplasm between the plasmalemma and the desmotubule.
Plastid.	 Organelle with a double membrane in the cytoplasm of many eukaryotes. May be concerned with photosynthesis (chloroplast) or starch storage (amyloplast), or contain yellow or orange pigments (chromoplast).
Plastochron (or plastochrone).	 The time interval between the inception of two successive repetitive events, as origin of leaf primordia, attainment of certain stage of development of a leaf, etc. Variable in length as measured in time units.
Plastoglobule.	 Globule in a plastid with lipid as the basic component.
Plate collenchyma.	 See lamellar collenchyma.
Plate meristem.	 A meristematic tissue consisting of parallel layers of cells dividing only anticlinally with reference to the wide surface of the tissue. Characteristic of ground meristem of plant parts that assume a flat form, as a leaf.
Plerome.	 The meristem forming the core of the axis composed of the primary vascular tissues and associated ground tissue, such as pith and interfascicular regions. One of the three histogens, plerome, periblem, and dermatogen, according to Hanstein.
Plicate mesophyll cell.	 A mesophyll cell with folds or ridges of cell wall projecting into cell lumen.
Plumule.	 Embryonic shoot above the cotyledon or cotyledons in an embryo. See also epicotyl.
Polar nucleus.	 One of two nuclei in the central cell of a mature embryo sac. The two nuclei are derived from groups of nuclei at the two opposite poles of the eight-nucleate embryo sac.
Pollen.	 A collective term for pollen grains.
Pollen conducting tissue.	 See transmitting tissue.
Pollen grain.	 A microspore in a seed plant included in an elaborately structured wall (one cell). Also a germinated microspore having formed a microgametophyte, immature (two cells) or mature (three cells).
Pollen mother cell.	 See microsporocyte.
Pollen sac.	 A locule in the anther containing the pollen grains.
Pollen tube.	 A tubular cell I extension formed by the germinating pollen grain; carries the male gametes into the ovule.
Pollination, The transfer of pollen from the anther to the receptive surface, stigma in angiosperms.	
Polyarch.	 Primary xylem of root; having many protoxylem strands, or protoxylem poles.
Polyderm.	 A type of protective tissue in which suberized cells alternate with nonsuberized parenchyma cells and both kinds of cell have living protoplasts.
Polyembryony.	 Development of more than one embryo in a single seed.
Polymerization.	 Chemical union of monomers, such as glucose or nucleotides, resulting in the formation of polymers, such as starch, cellulose, or nucleic acid.
Polyribosome (or polysome).	 Aggregation of ribosomes apparently concerned with protein synthesis as a group.
Polysaccharide.	 A carbohydrate composed of many monosaccharide units joined in a chain, e.g., starch, cellulose.
Pore.	 A term of convenience for the cross section of a vessel in the secondary xylem.
Pore cluster.	 See pore multiple.
Pore multiple.	 In secondary xylem; a group of two or more pores (cross sections of vessels) crowded together and flattened along the sur
faces of contact.	 Radial pore multiple, pores in radial file; pore cluster, irregular grouping.
Porous wood.	 Secondary xylem having vessels.
P-protein.	 Phloem protein. Found in cells of phloem of seed plants, most commonly in sieve elements. Formerly called slime.
Precursory cell.	 A cell giving rise to others by division. Mother cell.
Preprophase band.	 A ringlike band of microtubules delimiting the equatorial plane of the future mitotic spindle in cells preparing to divide.
Primary body (of plant).	 The part of the plant, or entire plant if no secondary growth occurs, that arises from the embryo and the apical meristems and their derivative meristematic tissues and is composed of primary tissues.
Primary cell wall.	 Version based on studies with the light microscope: cell wall formed chiefly while the cell is increasing in size. Version based on studies with the electron microscope: cell wall in which the cellulose microfibrils show various orientations-from random to more or less parallel-that may change considerably during the increase in size of the cell. The two versions do not necessarily coincide in delimiting primary from secondary wall.
Primary endosperm nucleus.	 Nucleus resulting from the fusion of the male gamete and two polar nuclei in the central cell of the embryo sac.
Primary growth.	 The growth of successively formed roots and vegetative and reproductive shoots from the time of their initiation by the apical meristems and until the completion of their expansion. Has its inception in the apical meristems and continues in their derivative meristems, protoderm, ground meristem, and procambium, as well as in the partly differentiated primary tissues.
Primary meristem.	 Often used for each of the three meristematic tissues derived from the apical meristem: protoderm, ground meristem, and procambium.
Primary phloem.	 Phloem tissue differentiating from procambium during primary growth and differentiation of a vascular plant. Commonly divided into the earlier protophloem and the later metaphloem. Not differentiated into axial and ray systems.
Primary phloem fibers.	 Fibers located on the outer periphery of the vascular region and originating in the primary phloem, usually the protophloem. Often called pericyclic fibers.
Primary pit.	 See primary pit-field.
Primary pit-field.	 A thin area of the primary cell wall and middle lamella within the limits of which one or more pit-pairs develop if a secondary wall is formed. Also called primordial pit and primary pit.
Primary plant body.	 See primary body.
Primary root.	 The taproot. Root developing in continuation of the radicle of the embryo.
Primary thickening meristem.	 A meristem derived from the apical meristem and responsible for the primary increase in thickness of the shoot axis. May appear as a distinct mantlelike zone. Often found in monocotyledons.
Primary tissues.	 Tissues derived from the embryo and the apical meristems.
Primary vascular tissues.	 Xylem and phloem differentiating from procambium during primary growth and differentiation of a vascular plant.
Primary wall.	 See primary cell wall.
Primary xylem.	 Xylem tissue differentiating from procambium during primary growth and differentiation of a vascular plant. Commonly divided into the earlier protoxylem and the later metaxylem. Not differentiated into axial and ray systems.
Primordial pit.	 See primary pit-field.
Primordium (pl.	 primordia). An organ, a cell, or an organized series of cells in their earliest stage of differentiation, e.g., leaf primordium, sclereid primordium, vessel member primordium.
Procambium.	 Primary meristem or meristematic tissue which differentiates into the primary vascular tissue. Also called provascular tissue.
Procumbent ray cell.	 In secondary vascular tissues; a ray cell having its longest axis in radial direction.
Prodesmogen.	 A meristem precursory to desmogen (procambium). The term has the same connotation as residual meristem.
Proembryo.	 Embryo in early stages of develop
ment, the stages before main body and suspensor become distinct or before the protoderm is initiated.	
Prokaryotic (also procaryotic).	 Refers to organisms in which the nuclear material is not enclosed in a nuclear envelope and the genetic material is not organized into chromosomes and which lacks membrane-bound cytoplasmic organelles. Blue-green algae, bacteria, mycoplasmas. Opposite to eukaryotic.
Promeristem.	 The initiating cells and their most recent derivatives in an apical meristem. Also called protomeristem.
Prop roots.	 Adventitious roots developing above the soil level on the stem and serving as additional support of the plant axis.
Prophyll.	 The first or one of two first leaves on a lateral shoot.
Proplastid.	 A plastid in its earliest stages of development.
Protective layer.	 In abscission zone; layer of cells that, because of substances impregnating its walls, has a protective function in the scar formed by abscission of a leaf or other plant part.
Protoderm.	 Primary meristem or meristematic tissue giving rise to the epidermis; also epidermis in meristematic state. May or may not arise from independent initials in the apical meristem.
Protomeristem.	 See promeristem.
Protophloem.	 The first-formed elements of the phloem in a plant organ. First part of the primary phloem.
Protophloem poles.	 Term of convenience for loci of phloem elements that are the first to mature in the vascular system of a plant organ. Applied to views in cross sections.
Protoplasm.	 Living substance. Inclusive term for all living contents of a cell or an entire organism.
Protoplast.	 The organized living unit of a single cell including protoplasmic and nonprotoplasmic contents of a cell but excluding the cell wall.
Protostele.	 The simplest type of stele, containing a solid column of vascular tissue, with the phloem peripheral to the xylem.
Protoxylem.	 The first formed elements of the xylem in a plant organ. First part of the primary xylem.
Protoxylem lacuna.	 Space surrounded by parenchyma cells in the protoxylem of a vascular bundle. Appears in some plants after the tracheary elements of protoxylem cease to function and are stretched and torn.
Protoxylem poles.	 Term of convenience for loci of xylem elements that are the first to mature in the vascular system of a plant organ. Applied to views in cross sections.
Provascular tissue.	 See procambium.
Proximal.	 Situated near the point of origin or attachment. Opposite of distal.
Pulvinus.	 An enlargement of the petiole of a leaf, or petiolule of a leaflet, at its base. A structure that has a role in the movements of a leaf or leaflet.
Quantasomes.	 Granules located on the inner surface of chloroplast membranes making up the thylakoids. Once thought to be functional units in photosynthesis.
Quarter-sawed oak.	 Oak wood sawed along a radial plane so that the radial surface showing the wide rays characteristic of this wood is exposed.
Quiescent center.	 The initial region in the apical meristem that has reached a state of relative inactivity; common in roots.
Radial parenchyma.	 See ray parenchyma.
Radial pore multiple.	 See pore multiple.
Radial section.	 A longitudinal section coinciding with a radius of a cylindrical body, such as stem.
Radial seriation.	 Arrangement of units, such as cells, in an orderly sequence in a radial direction. Characteristic of cambial derivatives.
Radial symmetry.	 Refers to a flower that can be divided in two equal parts in more than one longitudinal plane passing through the floral axis. Contrasted with bilateral symmetry.
Radial system.	 See ray system.
Radicle.	 The embryonic root. Forms the basal continuation of the hypocotyl in an embryo.
Ramified.	 Branched.
Ramiform pit.	 Pit that appears to be branched because it is formed by a coalescence of two or
more simple pits during the increase in thickness of the secondary wall.	
Raphe.	 A ridge along the body of the seed formed by the part of the funiculus that is adnate to the ovule (in an anatropous ovule).
Raphides.	 Needle-shaped crystals commonly occurring in bundles.
Ray.	 A panel of tissue variable in height and width, formed by the ray initials in the vascular cambium and extending radially in the secondary xylem and secondary phloem.
Ray initial.	 A meristematic ray cell in the vascular cambium that gives rise to ray cells of the secondary xylem and seconary phloem.
Ray parenchyma.	 Parenchyma cells of a ray in secondary vascular tissues. Contrasted with axial parenchyma.
Ray system.	 The total of all rays in the secondary vascular tissues. Also called horizontal system and radial system.
Ray tracheid.	 Tracheid in a ray. Found in the secondary xylem of certain conifers.
Reaction wood.	 Wood with more or less distinctive anatomical characteristics formed in parts of leaning or crooked stems and on lower (conifers) or upper (dicotyledons) sides of branches. See compression wood and tension wood.
Receptacle.	 The part of the flower stalk that bears the floral organs.
Redifferentiation.	 A reversal in differentiation in a cell or tissue and subsequent differentiation into another type of cell or tissue.
Regular flower.	 See radial symmetry.
Reproductive apical meristem.	 See apical meristem.
Residual meristem.	 Used in the sense of residuum of the least differentiated part of the apical meristem. A tissue that is relatively more highly meristematic than the associated differentiating tissues beneath the apical meristem. Gives rise to procambium and to interfascicular ground tissue.
Resin canal.	 See resin duct.
Resin duct.	 A duct of schizogenous origin lined with resin-secreting cells (epithelial cells) and containing resin.
Reticulate cell wall thickening.	 In tracheary elements of the xylem; secondary cell wall deposited on the primary so as to form a netlike pattern.
Reticulate perforation plate.	 In vessel member of the xylem; a type of mulitperforate plate in which the bars delimiting the perforations form a netlike pattern.
Reticulate sieve plate.	 A compound sieve plate with sieve areas arranged so as to form a netlike pattern.
Reticulate venation.	 Veins in leaf blade form an anastomosing system, the whole resembling a net. Netted venation
Reticulum.	 A net.
Retting.	 Freeing fiber bundles from other tissues by utilizing the action of microorganisms causing, in a suitable moist environment, the disintegration of the thin-walled cells surrounding the fibers.
Rexigenous.	 As applied to an intercellular space, orginating by rupture of cells.
Rhizodermis.	 Primary surface layer of the root. Use of the term implies that this layer is not homologous with the epidermis of the shoot. See also epiblem.
Rhytidome.	 A technical term for the outer bark, which consists of periderm and tissues isolated by it, namely cortical and phloem tissues.
Rib.	 An elongated protrusion as those along the large veins on the underside of a leaf.
Rib meristem.	 A meristematic tissue in which the cells divide perpendicular to the longitudinal axis of an organ and produce a complex of parallel, vertical files ("ribs") of cells. Particularly common in ground meristem of organs assuming a cylindrical form. Also called file meristem.
Ribosome.	 A cell component (sometimes called organelle) composed of protein and RNA and concerned with protein synthesis. Occurs in the cytoplasm, nucleus, plastids, and mitochondria.
Ring bark.	 A type of rhytidome resulting from the formation of successive periderms approximately concentrically around the axis.
Ring-porous wood.	 Secondary xylem in which the pores (vessels) of the early wood are distinctly larger than those of the late wood and form a well-defined zone or ring in a cross section of wood.
Rootcap.	 A thimblelike mass of cells covering the apical meristem of the root.
Root hair.	 A type of trichome on root epidermis that is a simple extension of an epidermal cell and is concerned with absorption of soil solution.

Sapwood.	 Outer part of the wood of stem or root containing living cells and reserves and in which conduction of water takes place. Generally lighter colored than the heartwood
Scalariform cell wall thickening.	 In tracheary elements of the xylem; secondary wall deposited on the primary so as to form a ladderlike pattern. Similar to a helix of low pitch with the coils interconnected at intervals.
Scalariform perforation plate.	 In vessel member of the xylem; a type of multiperforate plate in which elongated perforations are arranged parallel to one another so that the cell wall bars between them form a ladderlike pattern.
Scalariform pitting.	 In tracheary elements of the xylem; elongated pits arranged parallel to one another so as to form a ladderlike pattern.
Scalariform-reticulate cell wall thickening.	 In tracheary elements of the xylem; secondary thickening intermediate between scalariform and reticulate.
Scalariform sieve plate.	 A compound sieve plate with elongated sieve areas arranged parallel to one another in a ladderlike pattern.
Scale bark.	 A type of rhytidome in which the sequent periderms develop as restricted overlapping strata, each cutting out a scalelike mass of tissue.
Scar tissue.	 Composed of necrosed cells injured by wounding and subjacent cells impregnated with protective substances. See also cicatrice.
Schizogenous.	 As applied to an intercellular space, originating by separation of cell walls along the middle lamella.
Schizo-lysigenous.	 As applied to an intercellular space, originating by a combination of two processes, separation and degradation of cell walls.
Sclereid.	 A sclerenchyma cell, varied in form, but typically not much elongated, and having thick lignified secondary walls with many pits.
Sclerenchyma.	 A tissue composed of sclerenchyma Also a collective term for sclerenchyma in the plant body or plant organ. Includes fibers, fiber-sclerids, and sclereids.
Sclerenchyma cell.	 Cell variable in form and size ing more or less thick, often lignified, secondary walls. Belongs to the category of subcells and may or may not be devoid of protoplast at maturity.
Sclerification.	 The act of becoming changed into ,sclerenchyma, that is, developing secondary walls with or without subsequent lignification. 
Sclerotic parenchyma cell.	 A parenchyma cell that through deposition of a thick secondary wall is changed into a sclereid.
Scutellum(pl.	 scutella). The cotyledon in Poaceae embryo specialized for absorption of endosperm.

Secondary body.	 The part of the plant body that is to the primary body by the activity of the meristems, vascular cambium and phellogen. Consists of secondary vascular tissues and periderm.
Secondary cell wall.	 Version based on studies with the light microscope: cell wall deposited in some cells over the primary wall after the primary wall ceases to increase in surface. Version based on studies with the electron microscope: cell wall in which the cellulose microfibrils show a definite parallel orientation. The two versions do not necessarily coincide in delimiting secondary from primary wall.
Secondary endosperm nucleus.	 Nucleus resulting from fusion of two polar nuclei in the central cell of the embryo sac of angiosperms.
Secondary growth.	 In gymnosperms, most dicotyledons and some monocotyledons. A type of growth characterized by an increase in mass of stem and root and resulting from formation of  secondary vascular tissues by the vascular cambium. Commonly supplemented by of the cork cambium (phellogen) forming periderm
Secondary phloem.	 Phloem tissue formed by the vascular cambium during secondary growth in a vascular plant. Differentiated into axial and ray systems.
Secondary phloem fiber.	 A fiber located in the axial system of secondary phloem.
Secondary plant body.	 See secondary body.
Secondary root.	 See branch root.
Secondary thickening.	 Used for both deposition of secondary cell wall material and secondary increase in thickness of stems and roots.
Secondary tissues.	 Tissues produced by vascular cambium and phellogen during secondary g rowth.
Secondary vascular tissues.	 Vascular tissues (both xylem and phloem) formed by the vascular cambium during secondary growth in a vascular plant. Differentiated into axial and ray systems.
Secondary wall.	 See secondary cell wall.
Secondary xylem.	 Xylem tissue formed by the vascular cambium during secondary growth in a vascular plant. Differentiated into axial and ray systems.
Secretory cavity.	 Commonly refers to a space lysigenous in origin and containing a secretion derived from the cells that broke down in the formation of the cavity.
Secretory cell.	 A living cell specialized with regard to secretion or excretion of one or more, often organic, substances.
Secretory duct.	 Commonly refers to a duct schizogenous in origin and containing a secretion derived from the cells (epithelial cells) lining the duct. See epithelium.
Secretory hair.	 See glandular hair.
Secretory structure.	 Any of a great variety of structures, simple or complex, external or internal, that produces a secretion.
Seed coat.	 The outer coat of the seed derived from the integument or integuments. Also called testa.
Seminal adventitious root.	 Root initiated in the embryo on the hypocotyl or higher on the axis.
Sepal.	 A unit of the calyx.
Separation layer.	 See abscission layer.
Septate fiber.	 A fiber with thin transverse walls (septa), which are formed after the cell develops a secondary wall thickening.
Septum (pl.	 septa). A partition.
Sessile.	 Refers to a leaf lacking a petiole or a flower or a fruit lacking a pedicel. 
Sexine.	 The outer layer of the exine of a pollen grain. Sculptured part of exine.
Sheath.	 A sheetlike structure enclosing or encircling another. Applied to tubular or enrolled part of an organ, such as a leaf sheath, and to a tissue layer surrounding a complex of another tissue, as a bundle sheath enclosing a vascular bundle.
Sheathing base.	 Applied to a leaf base that encircles the stem.
Shell zone.	 In axillary bud primordia; a zone of parallel curving layers of cells, the entire complex shell-like in form. A result of orderly cell division along the proximal limits of the primordium.
Sieve area.	 A pitli ke area in the wall of a sieve element with pores commonly lined with callose arid occupied by protoplasmic material that interconnects the protoplasts of contiguous sieve elements.
Sieve cell.	 A type of sieve element that has relatively undifferentiated sieve areas (with narrow pores), rather uniform in structure on all walls; that is, there are no sieve plates. Typical of gymnosperms and lower vascular plants.
Sieve element.	 The cell in the phloem tissue concerned with mainly longitudinal conduction of food materials. Classified into sieve cell and sieve tube member.
Sieve field.	 Old term for a relatively undifferentiated sieve area found on wall parts other than the sieve plates.
Sieve pitting.	 An arrangement of small pits in sievelike clusters.
Sieve plate.	 The part of the wall of a sieve element bearing one or more highly differentiated sieve areas. Typical of angiosperms.
Sieve tube.	 A series of sieve elements (sieve tube members) arranged end to end and interconnected through sieve plates.
Sieve tube element.	 See sieve tube member.
Sieve tube member.	 One of the series of cellular components of a sieve tube. It shows a more or less pronounced differentiation between sieve plates (wide pores) and lateral sieve areas (narrow pores). Also sieve tube element and the obsolete sieve tube segment.
Silica cell.	 Cell filled with silica, as in epidermis of grasses.
Simple laticifer.	 Laticifer that is a single cell. Non articulated laticifer.
Simple perforation plate.	 In vessel member of the xylem; a perforation plate with a single perforation.
Simple pit.	 A pit in which the cavity becomes wider, remains of constant width, or only gradually becomes narrower during the growth in thickness of the secondary wall, that is, toward the lumen of the cell.
Simple pit-pair.	 An intercelluar pairing of two simple pits.
Simple sieve plate.	 Sieve plate composed of one sieve area.
Siphonostele.	 A type of stele in which the vascular system appears in the form of a hollow cylinder; that is, pith is present.
Slime.	 See P-protein.
Slime body.	 An aggregation of P-protein.
Slime plug.	 An accumulation of P-protein on a sieve area, usually with extensions into the sieve area pores.
Softwood.	 Technical designation of the wood of conifers.
Solitary pore.	 A pore (cross section of a vessel in secondary xylem) surrounded by cells other than vessel members.
Specialization.	 Change in structure of a cell, a tissue, plant organ, or entire plant associated with a restriction of functions, potentialities, or adaptability to varying conditions. May result in greater efficiency with regard to certain specific functions. Some specializations are irreversible, others reversible.
Specialized.	 Refers to (1) organisms having special adaptations to a particular habitat or mode of life; (2) cells or tissues having a characteristic function distinguishing them from other cells or tissues, more generalized in their function.
Spherosomes.	 Spherical bodies in the cytoplasm containing mostly lipid. Limiting membrane is thought to be absent or consisting of a unit membrane or of half of a unit membrane.
Spindle fibers.	 Microtubules aggregated in a spindle-shaped complex extending from pole to pole in a cell with a dividing nucleus. Term refers to light microscope views.
Spiral cell wall thickening.	 See helical cell wall thickening.
Spongy parenchyma.	 Leaf mesophyll parenchyma with conspicuous intercellular spaces.
Sporophyll.	 A modified leaf or leaflike organ that bears sporangia. In angiosperms, refers to stamens and carpels.
Sporopollenin.	 The substance composing the outer wall, or exine, of the pollen grain or a spore. A cyclic alcohol highly resistant to decay.
Spring wood.	 See early wood.
Square ray cell.	 In secondary vascular tissues, a ray cell approximately square as seen in radial section. (Considered to be of the same morphological type as the upright ray cell.)
Stamen.	 Floral organ producing the pollen and usually composed of anther and filament. The stamens together constitute the androecium.
Staminate.	 Refers to a flower having stamens but no functional carpels.
Starch.	 An insoluble carbohydrate, the chief food storage substance of plants, composed of anhydrous glucose residues of the formula C,H,o05 into which it easily breaks down.
Starch sheath.	 Applied to the innermost region (one or more cell layers) of the cortex when this region is characterized by conspicuous and rather stable accumulation of starch.
Stele (column).	 Conceived by Van Tieghem as a morphologic unit of the plant body comprising the vascular system and the associated ground tissue (pericycle, interfascicular regions, and pith). The central cylinder of the axis (stem and root).
Stellate.	 Star shaped.
Stereom (or stereome).	 Collective term for supporting tissue as contrasted with the conducting tissues hadrom and leptom.
Stigma.	 The region of the carpel, in many taxa at the apex of the style, that serves as a surface upon which the pollen germinates.
Stigmatoid tissue.	 A tissue cytologically and physiologically rather similar to the tissue of the stigma and serving as a path for the pollen tube
in the style.	 Preferred designations, pollen conducting tissue and pollen transmitting tissue.
Stoma (pl.	 stomata). An opening in the epidermis of leaves and stems bordered by two guard cells and serving in gas exchange.
Stomatal complex.	 Stoma and associated epidermal cells that may be ontogenetically and/or physiologically related to the guard cells. Also called stomatal apparatus.
Stomatal crypt.	 A depression in the leaf, the epidermis of which bears stomata.
Stomium.	 A fissure or pore in the anther lobe through which the pollen is released. Its formation is a type of dehiscence.
Stone cell.	 See brachysclereid.
Storied cambium.	 Vascular cambium in which the fusiform initials and rays are arranged in horizontal tiers on tangential surfaces. Stratified cambium.
Storied cork.	 Protective tissue found in the monocotyledons. The suberized cells occur in radial files, each consisting of several cells all of which are derived from one cell.
Storied wood.	 Wood in which the axial cells and rays are arranged in horizontal tiers on tangential surfaces. (Rays alone may be storied.) Stratified wood.
Strasburger cells.	 See albuminous cells.
Stratified cambium.	 See storied cambium.
Stratified wood.	 See storied wood.
Striate venation.	 See parallel venation.
Stroma.	 The ground substance of plastids.
Stromacenter.	 Aggregated fibrils, each 85 angstroms in diameter and of uncertain length, found in the stroma of a chloroplast fixed with glutaraldehyde (or acrolein)-osmium tetroxide.
Style.	 Extension of the top of the ovary, usually columnar, through which the pollen tube grows.
Stylode.	 Stylar branch.
Styloid.	 An elongated crystal with pointed or square ends.
Subapical initial.	 A cell beneath the protoderm at the apex of a leaf primordium that appears to function as an initial of the interior tissue of the leaf. Questionable concept.
Suberin.	 Fatty substance in the cell wall of cork 
tissue and in the casparian strip of the endodermis.	
Suberization.	 Impregnation of the cell wall with suberin or deposition of suberin lamellae on the wall.
Submarginal initials.	 Cells beneath the protoderm along the margins of a growing leaf lamina that appear to contribute cells to the interior tissue of the leaf. Components of marginal meristem which is concerned with marginal growth.
Subsidiary cell.	 An epidermal cell associated with a stoma and at least morphologically distinguishable from the epidermal cells composing the groundmass of the tissue. Also called accessory cell.
Summer wood.	 See late wood.
Superior ovary.	 See hypogyny.
Supernumerary cambium layer.	 Vascular cambium originating in phloem or pericycle outside the regularly formed vascular cambium. Characteristic of some plants with anomalous type of secondary growth.
Supporting cell.	 See supporting tissue.
Supporting tissue.	 Refers to tissue composed of cells with more or less thickened walls, primary (collenchyma) or secondary (sclerenchyma), that adds strength to the plant body. Also called mechanical tissue.
Suspensor.	 An extension at the base of the embryo that anchors the embryo in the embryo sac and pushes it into the endosperm.
Symplastic growth.	 See coordinated growth.
Syncarpy.	 Condition in flower characterized by union of carpets.
Syndetocheilic.	 Stomatal type in gymnosperms; subsidiary cells (or their precursors) are derived from the same protodermal cell as the guard-cell mother cell.
Synergids.	 Two cells in the micropylar end of the embryo sac associated with the egg in the egg apparatus of angiosperms. Play a vital role in fertilization.
Tabular.	 Having the form of a tablet or slab.
Tangential.	 In the direction of the tangent; at right angles to the radius. May coincide with periclinal.
Tangential section.	 A longitudinal section cut at right angles to a radius. Applicable to cylindrical structures such as stem or root, but used also for leaf blades when the section is made parallel with the expanded surface. Substitute term for leaf, paradermal.
Tannin.	 General term for a heterogeneous group of phenol derivatives. Amorphous strongly astringent substance widely distributed in plants, and used in tanning, dyeing, and preparation of ink.
Tapetum.	 In anther, a layer of cells lining the locule and absorbed as the pollen grains mature. In ovule, integumentary epidermis next to the embryo sac; also called endothelium.
Taproot.	 The first, or primary, root of a plant forming a direct continuation of the radicle of the embryo.
Taproot system.	 A root system based on the taproot, which may have branches of various orders.
Tasche.	 In German, pocket. Covering of primordium of lateral root derived from the endodermis, as distinguished from the rootcap, which is derived from the pericycle.
Taxon (pl.	 taxa). Any one of the categories (species, genus, family, etc.) into which living organisms are classified.
Teichode.	 A linear space in the outer epidermal wall in which the fibrillar structure is more loose and open than elsewhere in the wall. Replaces the term ectodesma.
Telome.	 One of the distal branches of a dichotomized axis, a morphological unit in a primitive vascular plant.
Telome theory.	 A theory that regards the telomes as basic units from which the diverse types of leaves and sporophylls of the vascular plants have evolved.
Template.	 A pattern or mould guiding the formation of a negative or a complement. A term applied in biology to DNA duplication (template hypothesis).
Tension wood.	 Reaction wood in dicotyledons, formed on the upper sides of branches and leaning or crooked stems and characterized by lack of lignification and often by high content of gelatinous fibers. 
Tepal.	 A member of the kind of floral perianth that is not differentiated into calyx and corolla.
Terminal apotracheal parenchyma.	 See boundary apotracheal parenchyma.
Testa.	 The seed coat.
Tetrarch.	 Primary xylem of root; having four protoxylem strands, or protoxylem poles.
Thylakoids.	 Saclike membranous structures (cisternae) in a chloroplast combined into stacks (grana) and present singly in the stroma (stroma thylakoids or frets) as interconnections between grana.
Tissue.	 Group of cells organized into a structural and functional unit. Component cells may be alike (simple tissue) or varied (complex tissue).
Tissue system.	 A tissue or tissues in a plant or plant organ structurally and functionally organized into a unit. Commonly three tissue systems are recognized, dermal, vascular, and fundamental (ground tissue system).
Tonoplast.	 A single cytoplasmic membrane bordering the vacuole. A kind of unit membrane.
Torus (pl.	 tori). The central thickened part of the pit membrane in a bordered pit consisting mainly of middle lamella and two primary walls. Typical of bordered pits in conifers and some other gymnosperms.
Trabecula (pl.	 trabeculae). A rodlike or spoolshaped part of a cell wall extending radially across the lumen of a cell. In initials and derivatives of vascular cambium in seed plants.
Trachea.	 Old term for xylem vessel implying a resemblance to an animal trachea.
Tracheary element.	 General term for a waterconducting cell, tracheid or vessel member.
Tracheid.	 A tracheary element of the xlyem that has no perforations, as contrasted with a vessel member. May occur in primary and in secondary xylem. May have any kind of secondary wall thickening found in tracheary elements.
Transection.	 Transverse section.
Transfer cell.	 Parenchyma cell with wall ingrowths (or invaginations) that increase the surface of the plasmalemma. Appears to be specialized for short-distance transfer of solutes. Cells without wall ingrowths may also function as transfer cells. See intermediary cell.
Transfusion tissue.	 In gynmosperm leaves, a tissue surrounding or otherwise associated with the vascular bundle and composed of tracheids and parenchyma cells. See also accessory transfusion tissue.
Transfusion tracheid.	 A tracheid in transfusion tissue.
Transition region.	 A region in the plant axis where root and shoot are united and which shows primary structural characteristics transitional between those of stem and root. Best exhibited in seedlings.
Transition zone.	 With reference to an apical meristem, a zone of orderly dividing cells disposed about the inner limit of the promeristem or, more specifically, of the group of central mother cells. Is transitional between the apical meristem and the subapical primary meristematic tissues.
Transitional cell.	 See intermediary cell.
Transmitting tissue, or pollen transmitting tissue.	 The tissue in the style of a flower through which the pollen tube grows between the stigma and the ovarian cavity. Also called pollenconducting tissue.
Transverse division (of cell).	 With reference to cell, division perpendicular to the longitudinal axis of the cell. With reference to plant part, division of the cell perpendicular to the long axis of the plant part.
Transverse section.	 A cross section. Section taken perpendicular to the longitudinal axis of an entity. Also called transection.
Traumatic resin duct.	 A resin duct developing in response to injury.
Triarch.	 Primary xylem of root; having three protoxylem strands, or three protoxylem poles.
Trichoblast.	 Commonly used for a cell in root epidermis that gives rise to a root hair.
Trichome.	 An outgrowth from the epidermis. Trichomes vary in size and complexity and include hairs, scales, and other structures and may be glandular.
Trichosclereid.	 A type of branched sclereid, usually with hairlike branches extending into intercellular spaces.
Trilacunar node.	 In a stem, a node with three leaf gaps related to one leaf. 
Tropism.	 Refers to movement or growth in response to an external stimulus the site of which determines the direction of the movement or growth.
Tube cells.	 Elongated cells with lignified walls in the inner epidermis of pericarp of a caryopsis of Poaceae.
Tunica.	 Peripheral layer or layers in an apical meristem of a shoot with cells that divide in the anticlinal plane and thus contribute to the growth in surface of the meristem. Forms a mantle over the corpus.
Tunica-corpus concept.	 A concept of the organization of apical meristem of shoot according to which this meristem is differentiated into two regions distinguished by their method of growth: the peripheral, tunica, one or more layers of cells showing surface growth (anticlinal divisons); the interior, corpus, a mass of cells showing volume growth (divisions in various planes).
Two-trace unilacunar condition.	 Characteristic of a node in a stem in which two leaf traces pertaining to one leaf are associated with one leaf gap.
Tylose (pl.	 tyloses). In xylem, an outgrowth from a parenchyma cell (axial or one in a ray) through a pit cavity into a tracheary cell, partially or completely blocking the lumen of the latter. Growth is preceded by a deposition of a special wall layer on the side of the parenchyma cell that forms the wall of the tylose.
Tylosoid.	 An outgrowth resembling a tylose. Examples are outgrowths of parenchyma cells into sieve elements in phloem and of epithelial cells into intercellular resin ducts.
Undifferentiated.	 In ontogeny, still in a meristematic state or resembling meristematic structures. In a mature state, relatively unspecialized.
Unifacial leaf.	 A leaf having similar structure on both sides. Conceived ontogenetically, a leaf that develops from a growth center abaxial or adaxial to the initial leaf primordium apex and thus includes tissues only from the abaxial or adaxial side of the primordium. The validity of the ontogenetic concept is questionable. Compare with bifacial leaf.
Unilacunar node.	 In a stem, a node with one leas gap related to one leaf. If two or more leaves are attached at such a node, each is associated with one gap.
Uniseriate ray.	 In secondary vascular tissues, ray one cell wide.
Unisexual.	 Usually refers to a flower lacking stamens or carpets. A perianth may be present or absent.
Unit membrane.	 A historical concept of basic membrane structure visualizing two layers of protein enclosing an inner layer of lipid, the three layers forming a unit. The term continues to be useful for describing sectioned membranes (profiles), exhibiting two dark lines separated by a clear space, as seen with the electron microscope.
Upright ray cell.	 In secondary vascular tissues, ray cell oriented axially (vertically in the axis) with its longest dimension.
Vacuolar membrane.	 See tonoplast.
Vacuolation.	 Ontogenetically, the development of vacuoles in a cell; in mature state, the presence of vacuoles in a cell.
Vacuole.	 Cavity within the cytoplasm filled with a watery fluid, the cell sap, and bound by a unit membrane, the tonoplast. Involved in uptake of water during germination and growth and maintenance of water in the cell. Also contains hydrolytic enzymes and has a lytic function.
Vacuome.	 Collective term for the total of all vac uoles in a cell, tissue, or plant.
Vascular.	 Refers to plant tissue or region consisting of or giving rise to conducting tissue xylem and/or phloem.
Vascular bundle.	 A strandlike part of the vascular system composed of xylem and phloem.
Vascular cambium.	 Lateral meristem that form; the secondary vascular tissues, secondary phloem em and secondary xylem, in stem and root. I located between those two tissues and, by pert clinal divisions, gives off cells toward boll tissues.
Vascular cylinder.	 Vascular region of the axis Term used synonymously with stele or centre cylinder or in a more restricted sense excluding the pith.
Vascular meristem.	 General term applicable to procambium and vascular cambium.
Vascular ray.	 A ray in secondary xylem or secondary phloem.
Vascular system.	 The total of the vascular tissues in their specific arrangement in a plant or plant organ.
Vascular tissue.	 A general term referring to either or both vascular tissues, xylem and phloem.
Vasicentric paratracheal parenchyma.	 Axial parenchyma in secondary xylem forming complete sheaths around vessels. See also paratracheal parenchyma.
Vegetative apical meristem.	 See apical meristem.
Vein.	 A strand of vascular tissue in a flat organ, as a leaf. Hence, leaf venation.
Vein rib.	 In a leaf, ridge of ground tissue occurring along a larger vein, usually on the lower side of the leaf.
Velamen.	 A multiple epidermis covering the aerial roots of some tropical epiphytic orchids and aroids. Occurs in some terrestrial roots also.
Venation.	 The arrangement of veins in the leaf blade.
Vertical parenchyma.	 See axial parenchyma.
Vertical system.	 In secondary vascular tissues. See axial system.
Vessel.	 A tubelike series of vessel members the common walls of which have perforations.
Vessel element.	 See vessel member.
Vessel member.	 One of the cellular components of a vessel. Also vessel element and the obsolete vessel segment.
Vestured pit.	 Bordered pit with projections from the overhanging secondary wall on the side facing the cavity.
Wall.	 See cell wall.
Water vesicle.	 A type of trichome. An enlarged highly vacuolated epidermal cell.
Wood.	 Usually secondary xylem of gymnosperms
and dicotyledons, but also applied to any other xylem.	
Wound cork.	 See wound peridem.
Wound gum.	 Gum formed as a result of some injury. See also gum.
Wound periderm.	 Periderm formed in response to wounding or other injury.
Xeromorphic.	 Refers to structural features typical of xerophytes.
Xerophyte.	 A plant adapted to a dry habitat.
Xylary procambium.	 The part of procambium that differentiates into primary xylem. Also called xylic or xyloic procambium.
Xylem.	 The principal water-conducting tissue in vascular plants characterized by the presence of tracheary elements. The xylem may also serve as a supporting tissue, especially the secondary xylem (wood).
Xylem elements.	 Cells composing the xylem tissue.
Xylem fiber.	 A fiber of the xylem tissue. Two types are recognized in the secondary xylem, fibertracheid and libriform fiber.
Xylem initial.	 A cambial cell on the xylem side of the cambial zone that is the source of one or more cells arising by periclinal divisions and differentiating into xylem elements either with or without additional divisions in various planes. Sometimes called xylem mother cell.
Xylem mother cell.	 A cambial derivative that is the source of certain elements of the xylem, such as axial parenchyma cells forming a parenchyma strand. Used also in a wider sense synonymously with xylem initial.
Xylem ray.	 That part of a vascular ray which is located in the secondary xylem.
Xylotomy.	 The anatomy of xylem.
Zygomorphic.	 Irregular flower. See bilateral symmetry Opposite of actinomorphic.