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GLOSSARY

How this glossary is organized

This entries in this glossary are not arranged alphabetically, but are instead grouped according to related concept. If you want to locate specific words, use your browsers "Search" or "Find" command.

Contents

Levels of Organization in the Plant Body
Cells
Tissues/tissue systems
Organs
Organisms
Plant Cells
Organelles
Vacuoles
Cell walls
Tissues and Cell Types in the Plant Body
Ground tissue system
Dermal tissue system
Vascular tissue system
Developmental Terminology
Meristems
Primary meristem
Secondary meristem
Primary meristematic tissue
The Organization of Roots, Stems, and Leaves
Vascular bundles
Leaves
Roots and stems
Flowers and fruits
Environmental Modifications
Organ Specialization
Wood and Bark





LEVELS OF ORGANIZATION IN THE PLANT BODY

cells: the smallest living unit of an organism. The outer boundary of a plant cells is defined by a rigid cell wall
tissues/tissue systems: groups of cells that share a similar function, such as transport (vascular tissue) or protection (dermal tissue)
organs:
root: the portion of a plant axis produced by the root apical meristem.
stem: the portion of a plant axis produced by the shoot apical meristem
leaf: a lateral appendage of the stem produced by the shoot apical meristem.
floral organs: modified leaves specialized for reproduction.
organism: any individual living creature, either multicellular or unicellular



PLANT CELLS

organelle: (literally "little organ") a region within a cell where specialized metabolic tasks occur; typically surrounded by a membrane.
plastids: a group of organelles characterized by a double membrane envelope and a complex of internal membranes. Plastids contain DNA and replicate autonomously.
proplastids: specialized for dividing to form new plastid, usually found in meristematic cells.
chloroplast: contains chlorophyll, internal membranes organized as grana, specialized for photosynthesis.
etioplast: a chloroplast that develops in the absence of light.
chromoplast: contains red, orange, or yellow carotenoid
pigments that are water insoluble, impart color to fruits, etc.
amyloplast: contains large amounts of starch, but no chlorophyll, specialized for storage.
elaioplast: contains oil droplets, usually found in fruits or seeds.
vacuole: membrane bound organelle that typically occupies a large volume of the cell cytoplasm. Vacuoles may contain:
anthocyanins: red, blue or purple pigments that are water soluble.
tannins: phenolic compounds than complex with protein; function in plant defense.
crystals: usually composed of calcium oxalate. Types are classified according to shape and include:
raphide crystals: needle-like, may inhibit herbivory.
druse crystals: granular.
cell wall: a rigid layer of cellulose and other polysaccharides, proteins and sometime lignin on the outside of the plasma membrane of a plant cell.
primary cell wall: a cell wall layer deposited while a cell is growing; typically extensible.
secondary cell wall: innermost layer of a cell wall deposited after cell enlargement has ceased, often lignified.
Casparian strip: a band of suberin within the anticlinal walls of endodermal and exodermal cells.
cuticle: a water repellent layer that coats the outer cell walls of the epidermis on aerial parts of plants, composed of cutin with a surface coating of wax.
mucigel: a slime sheath secreted by roots.
polysaccharide: a polymer composed of sugars.
cellulose: the structural (microfibrillar) portion of the plant cell wall. Cellulose is a polymer of glucose.
hemicellulose: the alkali-soluble portion of the cell wall matrix.
pectin: the hot-water-soluble portion of the cell wall matrix.
lignin: an aromatic polymer that rigidifies may secondary cell walls. Ligin is stained red by phloroglucinol solutions.
intercellular connections:
plasmodesma(ta): cytoplasmic channels lined with plasma membrane that connect the protoplasts of adjacent cells across the cell wall.
pit: a region where the secondary cell wall is absent, but the primary cell wall is present.
simple pit: a pit that is not bordered, may be round or slit-shaped.
circular-bordered pit: a round pit with a thickened margin.



TISSUES AND CELL TYPES OF THE PLANT BODY

ground tissue system: tissues derived from the ground meristem. All are simple tissues composed of a single type of cell, which is named after the tissue. The tissues of the ground tissue system include:
parenchyma: tissues composed of cells with thin primary cell wall. Types include:
chlorenchyma: contains chloroplasts and functions in photosynthesis.
aerenchyma: contains large intracellular air spaces and functions in gas exchange.
endodermis: characterized by a suberized Casparian strip; regulates transport of materials into the vascular bundles of most roots and some leaves and stems.
storage parenchyma: characterized by large accumulations of storage products such as starch, protein, oil, hemicellulose, or water.
collenchyma: tissues composed of cells with unevenly thickened primary cell walls that strengthen growing organs. Types are classified according to the arrangement of the wall thickenings and include.
angular collenchyma: cell wall is thickest in the corners.
lamellar collenchyma: cell wall is thickest on two opposite sides.
lacunar collenchyma: cell wall is thickest in the corners, intercellular air spaces present.
sclerenchyma: tissues composed of cells with thick, secondary cell wall that are usually lignified. Types are classified according to cell shape and include:
fiber: long, straight and thin, often occurring in bundles. Sometimes called "extraxylary fibers" to distinguish them from xylary fibers, which look similar, but have a different evolutionary origin.
sclereids: variable in shape, but not like fibers. Types are classified according to shape and include:
brachysclereids: also called stone cells, length and width nearly equal.
astrosclereids: star shaped, with several projecting arms.
trichosclereids: hair-like, similar to a fibers, except branched.
macrosclereids: column shaped, longer than wide.
osteosclereids: bone shaped, elongated with swollen ends.
secretory structures:
hydathode: a structure in the margins of leaves that secretes water.
oil cavities: a cavity lined with cells that secrete oils.
resin duct: a tube lined with cells that secrete resin.
laticifer: a secretory structure that produces latex.
latex: a milky fluid of unspecified composition.
dermal tissue system: Tissues derived from the protoderm or cork cambium that cover the surface of the plant body. The dermal tissues are complex (composed of several cell types) and include:
epidermis: a complex tissue that is usually a single cell layer thick and composed of the following cell types.
pavement cells: the least specialized cells of the epidermis (i.e. cells that are NOT specialized as guard cells, root hairs, trichomes, etc.). May secrete a cuticle.
guard cells: cells that surround and control the size of stomatal pores.
stomate (plural: stomata): an opening defined by pairs of guard cells that controls gas exchange and water loss.
subsidiary cells: cells adjacent to guard cells that are distinct in appearance from ordinary epidermal cells.
trichomes: cells that project from the surface of the epidermis. Types include:
unicellular trichome: consists of one cell.
multicellular trichome: consists of several cells.
secretory (glandular) trichome: secretes a substance.
root hair: specialized unicellular trichome found in roots.
Specialized epidermal cells and structures:
multiple epidermis: an epidermis that is more than one cell layer thick.
multistratose epidermis: a multiple epidermis in which all layers are derived from the protoderm.
hypodermis: a layer or layers of cells beneath the epidermis that is derived from the ground meristem, but distinct in appearance from adjacent ground tissue. May be called an endodermis if it has a Casparian strip.
velamen: a multistratose epidermis found in aerial roots.
bulliform cells: enlarged epidermal cells that facilitate leaf rolling in response to water stress.
lithocysts: literally translated "rock cells", cells containing a granule of calcium carbonate called a cystolith.
silica cells: cells in the epidermis of grasses that contain silica deposits.
nectary: a gland that secretes nectar.
vascular tissue system: Tissues derived from the procambium or vascular cambium that transport water and photosynthate. The vascular tissues are complex (composed of several cell types) and include:
xylem: the water-conducting tissue of plants.
tracheary element: a conducting cell of the xylem, characterized by an elongated shape and lignified secondary cell wall.
vessel element: a tracheary element with perforation plates.
perforation plate: the end wall of a vessel element where the secondary cell wall was not deposited and the primary cell wall has been digested.
foraminate: contains several round perforations.
scalariform: contains several elongated perforations such that the remaining cell wall resembles the rungs of a ladder.
simple: contains a single perforation.
vessel: a long tube of vessel elements connected by perforation plates
tracheid: a tracheary element that lacks perforations plates, water flows from between tracheids through pits.
fiber tracheid: a cell in the xylem that is intermediate between a tracheid and a libriform fiber.
libriform fiber: a cell in the xylem that is very long and thin and has simple pits, sometimes called "xylary fibers" to distinguish them from extraxylary fibers, which look similar, but have a different evolutionary origin.
phloem: the photosynthate-conducting tissue of plants.
sieve element: a conducting cell in the phloem.
sieve-tube member: a sieve element with perforation plates, characteristic of angiosperms.
sieve plate: the end wall of a sieve-tube element that is perforated by sieve plate pores.
sieve plate pore: an enlarged plasmodesma that perforates a sieve plate.
sieve tube: a long tube of sieve elements (also called sieve tube members) connected by sieve plates.
sieve cell: a sieve element that lacks perforation plates, characteristic of gymnosperms.
p-protein: a stringy protein within sieve elements that blocks sieve plate pores when the sieve tube is damaged.
companion cells: a cell in the phloem that is connected to a sieve-tube member by numerous plasmodesmata.
albuminous cells: a cell the phloem that is connected to a sieve cell by numerous plasmodesmata.



DEVELOPMENTAL TERMINOLOGY

indeterminant growth: a type of growth characterized by production of an unlimited number of organs.
differentiate: become specialized for a particular function
meristem: a defined region where new cells arise in predictable pattern; localized regions of cell division
primordium: a cell or organ in its initial stage of development.
leaf primordium: arises at the shoot apical meristem.
axillary bud primordium: arises in the axil of a leaf primordium.
lateral root primordium: arises in the pericycle.
adventitious: arising at an unexpected location.
Planes of cell division:
anticlinal: perpendicular to the surface.
periclinal: parallel to the surface.
phyllotaxy: the pattern of leaf initiation at the apical meristem.
plastochron: the time between the initiation of one leaf and the initiation of the next.
callus: an irregular proliferation of cells.



MERISTEMS

apex: the tip of something, usually a shoot, root or leaf.
meristematic region: a general zone in which cell division is frequent.
initial cell (=stem cell): a cell that, when it divides, leaves one of the daughter cells in the apical meristem.
apical initials: initial cells in the root or shoot apical meristem.
apical cell: the single initial in the apical meristems of most seedless vascular plants.
primary meristem: a meristem that is present in the embryo of a plant; generally responsible for increase in the length of plants
root apical meristem: a meristem located at the apex of a root.
root cap: a thimble-shaped mass of cells that covers the root apical meristem.
open: no distinct boundary separates the root cap from the root proper.
closed: root cap is distinct from the root proper.
shoot apical meristem: a meristem located at the apex of a shoot.
tunica: the outer layer(s) that divide only anticlinally.
corpus: the inner layers that divide anticlinally or periclinally.
apical dome: the part of the apical meristem interior to the leaf primordia.
secondary meristem: a meristem that arise from tissues produced by a primary meristem; generally responsible for increase in thickness of plants
vascular cambium: a sheet-like meristem that produces secondary xylem and secondary phloem.
residual procambium: procambium located between mature xylem and phloem.
fascicular cambium: arises within vascular bundles.
interfascicular cambium: arises between vascular bundles.
fusiform initial: an elongated cell in the vascular cambium, produces elements of the axial system.
ray initial: an isodiametric cell in the vascular cambium, produces elements of the ray system.
storied cambium: fusiform initials aligned with one another.
non-storied cambium: fusiform initials not aligned with one another.
cork cambium (phellogen): a sheet-like meristem that produces cork.
primary meristematic tissue: a group of cells beneath the apical meristem that has become distinct in appearance from neighboring groups of cells, a precursor to one of the tissue systems:
procambium: develops into the vascular tissue system.
protoderm: develops into the dermal tissue system.
ground meristem: develops into the ground tissue system.



THE ORGANIZATION OF ROOTS STEMS AND LEAVES

vascular bundle (vascular strand): a strand of tissue containing primary xylem and primary phloem.
axial bundle: a major vascular bundle in the shoot or root.
leaf trace: a vascular bundle that connects a leaf to the axial vascular system.
leaf gap: in a siphonostele, a break in the vascular cylinder above the point where a leaf trace arises.
vein: a vascular bundle in a leaf.
sympodium: an axial bundle and its leaf and branch trace.
Bundle types based on arrangement of xylem and phloem:
collateral bundle: contains a mass of phloem toward the exterior and a mass of xylem toward the interior
bicollateral bundle: contains one mass of xylem and two masses of phloem, one toward the interior and one toward the exterior.
stele: the arrangement of vascular bundles in roots and stems.
protostele: a single central vascular bundle.
polyarch: a protostele with many arms of xylem, most common in monocots.
siphonostele: a cylinder of vascular tissue with a central parenchymatous pith.
dictyostele: a siphonostele with numerous leaf gaps; superficially appears to be composed of vascular bundles.
eustele: a ring of vascular bundles surround a pith.
atactostele: a complex three dimensional network of vascular bundles; superficially bundles appear to be scattered.
leaves:
palisade mesophyll: the region of ground tissue in a leaf where the chlorenchyma cells are elongated and arranged perpendicular to the epidermis, usually in the upper half of the leaf.
spongy mesophyll: the region of ground tissue in a leaf where parenchyma cells are branched and intercellular air spaces are extensive, usually in the lower half of the leaf.
bundle sheath: the layer of tightly-packed cells that surround the vascular tissue in leaves.
bundle sheath extension: a group of cells that connect a vein to the epidermis in a leaf.
abscission zone: a region of the petiole
protective layer: the layer of cells that produce suberin to seal the petiole before abscission.
separation layer: the layer of cells that secrete cell wall degrading enzymes forming a weak point where an abscising leaf can drop off.
epidermis: the epidermis contains stomata arranged in one of the following ways:
amphistomatic: stomata in upper and lower epidermis.
epistomatic: stomata in the upper epidermis only.
hypostomatic: stomata in the lower epidermis only.
clustered stomata: stomata occur in groups.
sunken stomata: stomata located below the surface of the epidermis.
stomatal crypts: a pit containing several stomata.
roots and stems:
cortex: ground tissue between the vascular bundle and epidermis.
pith: ground tissue in the center of a stem.
pericycle: in roots, the layer of cells between vascular tissue and endodermis that gives rise to lateral roots and vascular cambium.
flowers, fruits and seeds:
Floral organs: all are modified leaves.
receptacle: the tip of a floral stem, supports the floral organs.
sepal: outermost and most leaf-like, usually encloses the rest of the flower in the bud.
petal: interior to sepals, usually conspicuous to attract pollinators.
stamen: interior to petals, produces the pollen.
filament: stalk-like portion of the stamen.
anther: pollen-bearing portion of the stamen.
pollen sac: one of four cavities in an anther that contain pollen.
pollen: the male gametophyte, includes two cells:
tube nucleus: located within the tube cell.
generative cell: divides to form two sperm.
tapetum: a layer of nutritive cells that lines the pollen sac.
pistil: innermost, bears the ovules, may occur singly or in clusters.
stigma: the portion of the pistil that receives the pollen.
style: the portion of the pistil through which the pollen tube grows.
ovary: the portion of the pistil that bears the ovules.
carpel: a unit of the pistil consisting of a single modified leaf, a simple pistil consists of one carpel and a compound pistil consists of fused carpels.
locule: a cavity containing ovules, in a compound ovary there is one locule per carpel.
ovule: an embryo sac with egg surrounded by nucellus and two integuments.
embryo sac: the eight-celled megagametophyte of flowering plants.
integuments: the outer protective layer of the ovule.
micropyle: an opening in the integuments through which a pollen tube enters.
nucellus: the inner nutritive layer of the ovule.
Classification of flowers by ovary position:
hypogynous: sepals, petals and stamens attached to the receptacle below the ovary.
epigynous: sepals, petals and stamens attached to the top of the ovary.
perigynous: sepals, petals and stamens attached to a hypanthium.
hypanthium: a cup-shaped extension of the receptacle.
Fruits and seeds:
seed: a mature ovule, includes:
embryo: a young plant present in the seed before germination.
radicle: the root portion of the embryo.
plumule: the shoot portion of the embryo.
cotyledon: the first leaves of an embryo, may or may not resemble true leaves.
coleorhiza: in monocots, a sheath that covers the radicle.
coleoptile: in monocots, a sheath the covers the plumule.
seed coat: the outermost layer of a seed, develops from the integuments.
endosperm: a triploid nutritive tissue that develops in the ovule, may be absorbed by the before the seed matures.
fruit: a mature ripened ovary. In practice, most plant parts that contains seeds are fruits.
funiculus: the stalk that attaches a seed to the inside of a fruit.



ENVIRONMENTAL MODIFICATION

xeromorphic: characters that are advantageous is dry environments.
hydromorphic: characters that are advantageous in wet environments.
halomorphic: characters that are advantageous in saline environments.
xerophyte: a plant with xeromorphic features.
mesophyte: a plant with neither hydromorphic nor xeromorphic features.
hydrophyte: a plant with hydromorphic features.
succulent: having large amounts of water storage tissue.
leaf dimorphism: leaves of two distinct types produced by the same plant, examples include:
sun/shade leaves: leaves in the sunnier parts of trees are distinct from more shaded leaves.
air/water leaves: in aquatic plants, leaves that grow below the water surface are distinct from those formed above the surface.
heteroblastic leaves: leaves produced during one stage of development are distinct from those formed at another stage, for example leaf from may change when a plant flowers.



ORGAN SPECIALIZATIONS

aerial root: a root that grows in air, common in epiphytes.
cataphyll (=bud scale): a leaf modified to protect a dormant bud.
cladode: a stem that is leaf-like in appearance.
contractile root: a root that contracts to pull the crown of the plant below the soil surface.
corm: an underground stem that is upright.
haustorial root: a root that is modified for absorbing water or nutrients from another plant.
mycorrhizae: a symbiotic relationship between plant roots and fungi.
endomycorrhizae: fungal mycelia are internal.
ectomycorrhizae: fungal mycelia are external
phyllode: a leaf that consists of an enlarged midrib and lacks blades.
prop roots: roots that help support a plant from above ground.
rhizome: an underground stem.
root nodules: structures that develop on the roots of plants that form symbiotic associations with nitrogen-fixing bacteria.
spine: a stem of leaf modified for protection.
storage root: a root modified to store relatively large amounts of food.
tendril: a stem of leaf modified to coil around other plants or objects.
trap: a leaf modified to trap insects to supply the plant with nitrogen.
tuber: a swollen underground stem, such as a potato



WOOD AND BARK

secondary xylem: water-conducting tissue produced by the vascular cambium, wood.
secondary phloem: photosynthate-conducting tissue produced by the vascular cambium, inner bark.
axial system: cells elongated parallel to the organ axis, develop from fusiform initials.
axial vessels: vessels of the axial system, may be:
clustered: several vessel occur in a bundle.
solitary: vessels separated by other types of cells.
axial parenchyma: parenchyma cells of the axial system.
ray system: cells elongated radially, develop from ray initials.
uniseriate rays: rays that are one cell wide.
multiseriate rays: rays that are more than one cell wide.
homocellular rays: rays consisting on one type of cell.
heterocellular rays: rays consisting of more that one type of cells, possibilities include:
ray parenchyma: parenchyma cells of the ray system, may be:
procumbent cells: elongated parallel to the ray.
upright cells: elongated perpendicular to the ray.
ray tracheids: tracheids of the ray system.
annual ring: the portion of a woody stem produced in one year.
diffuse porous: annual rings contain large vessel throughout.
ring porous: large vessels present only in inner portion of an annual ring.
anomalous secondary growth: secondary tissue that are not produced by a vascular cambium.
secondary vascular bundles: vascular bundles that do not develop from procambium.
periderm: the cork cambium and the tissues in produces, outer bark.
cork (phellem): cells produced by the cork cambium that have suberized cells walls and are dead at maturity.
lenticel: a region of the periderm where cells are loosely packed, allows gas exchange.

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Alison Roberts (aroberts@uri.edu)
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