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sugar sinka plant organ that is a net consumer or storer of sugar. Growing roots, shoot tips, stems, and fruits aresugar sinks supplied by phloem.

companion (transfer) cell

Found alongside each sieve tube element. Companion cells in leaves help load sugars into the sieve-

tube elements, which then transport the sugars to other parts of the plant.

nitrogen fixation

The conversion of atmospheric nitrogen (N2) to ammonia (NH3). Biological nitrogen fixation is carriedout by certain prokaryotes, some of which have mutualistic relationships with plants.

nitrogenase

Enzymes used by some organisms to fix atmospheric nitrogen gas (N2).

root nodules

Swellings composed of plant cells that have been infected by Rhyzobium.

mycorrhizae

A mutualistic association of plant roots and fungus.

sporophyte

In organisms that have alternation of generations, the multicellular diploid form that results from theunion of gametes. The sporophyte produces haploid spores by meiosis that develop into gametophytes.

gametophyte

In organisms that have alternation of generations, the multicellular haploid form that produces haploidgametes by mitosis. The haploid gametes unite and develop into sporophytes.

alternation of generationsA life cycle in which there is both a multicellular diploid form, the sporophyte, and a multicellularhaploid form, the gametophyte; characteristic of plants and some algae.

sepalA modified leaf in angiosperms that helps enclose and protect a flower bud before it opens.

stamenThe pollen-producing reproductive organ of a flower, consisting of an anther and a filament.

carpelThe ovule-producing reproductive organ of a flower, consisting of the stigma, style, and ovary.

pollen grain

In seed plants, a structure consisting of the male gametophyte enclosed within a pollen wall.

embryo sac

The female gametophyte of angiosperms, formed from the growth and division of the megaspore into amulticellular structure that typically has eight haploid nuclei.

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pollinationThe transfer of pollen to the part of a seed plant containing the ovules, a process required forfertilization.

endosperm

In angiosperms, a nutrient-rich tissue formed by the union of a sperm with two polar nuclei during

double fertilization. The endosperm provides nourishment to the developing embryo in angiospermseeds.

ovule

A structure that develops within the ovary of a seed plant and contains the female gametophyte.

double fertilization

A mechanism of fertilization in angiosperms in which two sperm cells unite with two cells in thefemale gametophyte (embryo sac) to form the zygote and endosperm.

fruitA mature ovary of a flower. The fruit protects dormant seeds and often aids in their dispersal.

imbibition

The physical adsorption of water onto the internal surfaces of structures.

vegetative reproduction

Cloning of plants by asexual means.

hormone

In multicellular organisms, one of many types of secreted chemicals that are formed in specializedcells, travel in body fluids, and act on specific target cells in other parts of the body to change theirfunctioning.

phototropism

Growth of a plant shoot toward or away from light.

auxinIndoleacetic acid (IAA), a natural plant hormone that has a variety of effects, including cell elongation,root formation, secondary growth, and fruit growth.

cytokinins

Any of a class of related plant hormones that retard aging and act in concert with auxin to stimulate celldivision, influence the pathway of differentiation, and control apical dominance.

gibberellin

Any of a class of related plant hormones that stimulate growth in the stem and leaves, trigger thegermination of seeds and breaking of bud dormancy, and (with auxin) stimulate fruit development.

abscisis acid

A plant hormone that slows growth, often antagonizing actions of growth hormones. Two of its manyeffects are to promote seed dormancy and facilitate drought tolerance.

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ethyleneThe only gaseous plant hormone. Among its many effects are response to mechanical stress,programmed cell death, leaf abscission, and fruit ripening.

gravitropism

A response of a plant or animal to gravity.

thigmotropism

A directional growth of a plant in response to touch.

photoperiodismA physiological response to photoperiod, the relative lengths of night and day. An example ofphotoperiodism is flowering.

short day plants

A plant that flowers (usually in late summer, fall, or winter) only when the light period is shorter than acritical length.

long day plants

A plant that flowers (usually in the late spring or early summer) only when the light period is longerthan a critical length.

Phytochromes

A type of light receptor in plants that mostly absorbs red light and regulates many plant responses, suchas seed germination and shade avoidance.