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abscisic acid. A stress hormone that inhibits the action of auxins and gibberellins.

abscission. The falling away of leaves, flowers, or fruits, usually following the formation of an abscission layer constituted of cork cells, absorption spectrum. Graph of absorption of varying wavelengths of light by a pigment.

abstriction. In ascomycetes, a form of conidiospore formation wherein spores are cut off from the ends of hyphae. acellular. Not being constituted of cells. In a mass of cytoplasm the nuclei are not separated from each other by membranes, achene. A one-seeded, dry, indehiscent fruit.

acrasin. A hormone produced in a myxomycete. Influences the coalescence of swarm spores.

acrasinase. An enzyme involved in the breakdown of the hormone acrasin. actinomorphic. Radially symmetrical. Star-shaped. Said of flowers, actinomycetes. A group of filamentous bacteria that reproduce by fission or conidia. Some cause diseases of plants and some produce antibiotics (e.g., Streptomyces). actinomycosis. An infection caused by an actinomycete. action spectrum. A measure of the rate of photosynthesis at varying wavelengths of light.

active transport. The movement of substances across a cell membrane.

Requires the expenditure of energy. ADP. Adenosine diphosphate.

adventitious. Characterized by a root arising from stem or a stem arising from a root.

aeciospores. Binucleated spores of a rust fungus and formed by a fusion of cells without the fusion of nuclei, aecium. In rust fungi, a cup-like structure that produces aeciospores. aerobic. Requiring oxygen for respiration.

akinete. An algal spore produced by a change in a vegetative cell, algin. A product derived from brown algae and used commercially (e.g., the manufacture of ice cream), aliphatic. An organic compound of open-chain structure (e.g., paraffin, fatty acids).

amino. Relating to the group -NIL.

amino acid. An organic acid in which hydrogen has been replaced by an amino group. It exhibits both acidic and basic properties, and is a building block in the manufacture of protein, amoeboid. Relating to changing shape, moving, or eating, or ingesting by means of pseudopodia (temporary cytoplasmic protrusions), amoeboid movement. Movement caused by flowing protoplasm accompanied by changing cell shape, amphimixis. The union of germ cells in sexual reproduction, anaerobic. Respiration accomplished in the absence of oxygen, anaphase. That phase in mitosis when chromatids separate and move toward opposite poles. In meiosis, the paired chromosomes move apart, androecium. A collective term for the stamens.

aneuploidy. Having a chromosome number that is not an exact multiple of the haploid number, n. angiosperm. The group of vascular plants having seeds enclosed in an ovary.

anisogamous. The fusion of unlike gametes, the difference usually being one of size.

annual. A plant that completes its life cycle in one growing season, annual ring. A growth layer that is seen in a cross section of a woody stem and that reveals the growth for a single year. The term applies primarily to wood grown in temperate zones, annuius. In ferns, the specialized ring of cells around the sporangium, anther. That part of the stamen consisting of pollen sacs that bear the pollen.

antheridiophore. The stalk on which an antheridium is borne, antheridium. A male gametangium (a spore-bearing organ of plants other than seed plants).

anthocyanin. Water-soluble pigments. Often found in central vacuoles, antibiotic. A substance that prevents or retards the growth of organisms. Often produced by microorganisms and prevents the growth of other microorganisms.

anticodon. The three nucleotide bases in m-RNA (the three bases in DNA

representing a codon). antipodal cells. The three cells of a mature embryo sac. Located at the end opposite the micropyle. apetalous. Lacking petals.

aphid. A small insect that sucks the juices of plants, causes wilting, may cause gall formation, and may serve as a vector for virus diseases of plants. Secretes a sweet liquid attractive to insects, apical meristem. In vascular plants, the dividing tissue located at the tip of a root or stem.

apogamy. The development of a sporophyte from the gametophyte without the union of gametes. The embryo arises from cells other than the egg. apomixis. Production of seed without the union gametes.

apothecium. In ascomycetes, a cup- or saucer-shaped structure bearing asci. archegoniophore. The stalk on which an archegonium is supported, archegonium. A female gametangium. A structure in which the egg is protected by a jacket of cells, ascogonium. The female, gamete-bearing structure of ascomycetes. ascospore. In ascomycetes, the spore produced within an ascus. ascus (plural, asci). In ascomycetes, the spore-bearing structure in which the ascospores are borne, asexual. A form of reproduction involving neither the union of gametes nor meiosis.

aspergillum. A structure used in dispersing holy water. The spore-bearing structure of Aspergillus is the same shape, hence the name. ATP. Adenosine triphosphate.

attachment site. In the t-RNA molecule, the site where an amino acid is attached so that it may be carried to the ribosome. autecology. A branch of ecology focusing on individual plants and their effects on the environment, autotrophic. Self-nourishing. Pertains to plants that manufacture their own nourishment.

auxin. A plant growth-regulating substance that allows cell elongation, avirulent. Lacking the capacity to cause disease.

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