abscisic acid (ABA)
plant hormone that induces dormancy in seeds and other organs
abscission
physiological process that leads to the fall of a plant organ (such as leaf or petal drop)
adventitious root
aboveground root that arises from a plant part other than the radicle of the plant embryo
apical bud
bud formed at the tip of the shoot
apical meristem
meristematic tissue located at the tips of stems and roots; enables a plant to extend in length
auxin
plant hormone that influences cell elongation (in phototropism), gravitropism, apical dominance and root growth
axillary bud
bud located in the axil: the stem area where the petiole connects to the stem
bark
tough, waterproof, outer epidermal layer of cork cells
bulb
modified underground stem that consists of a large bud surrounded by numerous leaf scales
Casparian strip
waxy coating that forces water to cross endodermal plasma membranes before entering the vascular cylinder, instead of moving between endodermal cells
chromophore
molecule that absorbs light
collenchyma cell
elongated plant cell with unevenly thickened walls; provides structural support to the stem and leaves
companion cell
phloem cell that is connected to sieve-tube cells; has large amounts of ribosomes and mitochondrion
compound leaf
leaf in which the leaf blade is subdivided to form leaflets, all attached to the midrib
corm
rounded, fleshy underground stem that contains stored food
cortex
ground tissue found between the vascular tissue and the epidermis in a stem or root
cryptochrome
protein that absorbs light in the blue and ultraviolet regions of the light spectrum
cuticle
waxy protective layer on the leaf surface
cuticle
waxy covering on the outside of the leaf and stem that prevents the loss of water
cytokinin
plant hormone that promotes cell division
dermal tissue
protective plant tissue covering the outermost part of the plant; controls gas exchange
endodermis
layer of cells in the root that forms a selective barrier between the ground tissue and the vascular tissue, allowing water and minerals to enter the root while excluding toxins and pathogens
epidermis
single layer of cells found in plant dermal tissue; covers and protects underlying tissue
ethylene
volatile plant hormone that is associated with fruit ripening, flower wilting, and leaf fall
fibrous root system
type of root system in which the roots arise from the base of the stem in a cluster, forming a dense network of roots; found in monocots
gibberellin (GA)
plant hormone that stimulates shoot elongation, seed germination, and the maturation and dropping of fruit and flowers
ground tissue
plant tissue involved in photosynthesis; provides support, and stores water and sugars
guard cells
paired cells on either side of a stoma that control stomatal opening and thereby regulate the movement of gases and water vapor
intercalary meristem
meristematic tissue located at nodes and the bases of leaf blades; found only in monocots
internode
region between nodes on the stem
jasmonates
small family of compounds derived from the fatty acid linoleic acid
lamina
leaf blade
lateral meristem
meristematic tissue that enables a plant to increase in thickness or girth
lenticel
opening on the surface of mature woody stems that facilitates gas exchange
megapascal (MPa)
pressure units that measure water potential
meristem
plant region of continuous growth
meristematic tissue
tissue containing cells that constantly divide; contributes to plant growth
negative gravitropism
growth away from Earth’s gravity
node
point along the stem at which leaves, flowers, or aerial roots originate
oligosaccharin
hormone important in plant defenses against bacterial and fungal infections
palmately compound leaf
leaf type with leaflets that emerge from a point, resembling the palm of a hand
parenchyma cell
most common type of plant cell; found in the stem, root, leaf, and in fruit pulp; site of photosynthesis and starch storage
pericycle
outer boundary of the stele from which lateral roots can arise
periderm
outermost covering of woody stems; consists of the cork cambium, cork cells, and the phelloderm
permanent tissue
plant tissue composed of cells that are no longer actively dividing
petiole
stalk of the leaf
photomorphogenesis
growth and development of plants in response to light
photoperiodism
occurrence of plant processes, such as germination and flowering, according to the time of year
phototropin
blue-light receptor that promotes phototropism, stomatal opening and closing, and other responses that promote photosynthesis
phototropism
directional bending of a plant toward a light source
phyllotaxy
arrangement of leaves on a stem
phytochrome
plant pigment protein that exists in two reversible forms (Pr and Pfr) and mediates morphologic changes in response to red light
pinnately compound leaf
leaf type with a divided leaf blade consisting of leaflets arranged on both sides of the midrib
pith
ground tissue found towards the interior of the vascular tissue in a stem or root
positive gravitropism
growth toward Earth’s gravitational center
primary growth
growth resulting in an increase in length of the stem and the root; caused by cell division in the shoot or root apical meristem
rhizome
modified underground stem that grows horizontally to the soil surface and has nodes and internodes
root cap
protective cells covering the tip of the growing root
root hair
hair-like structure that is an extension of epidermal cells; increases the root surface area and aids in absorption of water and minerals
root system
belowground portion of the plant that supports the plant and absorbs water and minerals
runner
stolon that runs above the ground and produces new clone plants at nodes
sclerenchyma cell
plant cell that has thick secondary walls and provides structural support; usually dead at maturity
secondary growth
growth resulting in an increase in thickness or girth; caused by the lateral meristem and cork cambium
sessile
leaf without a petiole that is attached directly to the plant stem
shoot system
aboveground portion of the plant; consists of non-reproductive plant parts, such as leaves and stems, and reproductive parts, such as flowers and fruits
sieve-tube cell
phloem cell arranged end to end to form a sieve tube that transports organic substances such as sugars and amino acids
simple leaf
leaf type in which the lamina is completely undivided or merely lobed
sink
growing parts of a plant, such as roots and young leaves, which require photosynthate
source
organ that produces photosynthate for a plant
statolith
(also, amyloplast) plant organelle that contains heavy starch granules
stele
inner portion of the root containing the vascular tissue; surrounded by the endodermis
stipule
small green structure found on either side of the leaf stalk or petiole
stolon
modified stem that runs parallel to the ground and can give rise to new plants at the nodes
strigolactone
hormone that promotes seed germination in some species and inhibits lateral apical development in the absence of auxins
tap root system
type of root system with a main root that grows vertically with few lateral roots; found in dicots
tendril
modified stem consisting of slender, twining strands used for support or climbing
thigmomorphogenesis
developmental response to touch
thigmonastic
directional growth of a plant independent of the direction in which contact is applied
thigmotropism
directional growth of a plant in response to constant contact
thorn
modified stem branch appearing as a sharp outgrowth that protects the plant
tracheid
xylem cell with thick secondary walls that helps transport water
translocation
mass transport of photosynthates from source to sink in vascular plants
transpiration
loss of water vapor to the atmosphere through stomata
trichome
hair-like structure on the epidermal surface
tuber
modified underground stem adapted for starch storage; has many adventitious buds
vascular bundle
strands of stem tissue made up of xylem and phloem
vascular stele
strands of root tissue made up of xylem and phloem
vascular tissue
tissue made up of xylem and phloem that transports food and water throughout the plant
venation
pattern of veins in a leaf; may be parallel (as in monocots), reticulate (as in dicots), or dichotomous (as in Gingko biloba)
vessel element
xylem cell that is shorter than a tracheid and has thinner walls
water potential (Ψw)
the potential energy of a water solution per unit volume in relation to pure water at atmospheric pressure and ambient temperature
whorled
pattern of leaf arrangement in which three or more leaves are connected at a node