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Plant Control Systems and Defenses


abscisic acid (ABA)

a plant growth hormone that controls dormancy and leaf aging


the natural separation of flowers, fruit, or leaves from the parent plant

apical dominance

the suppression of lateral bud development on growing shoots


a plant growth hormone that controls root growth, bud formation, and fruit maturity

axillary bud

a structure at the axil (angle between a leaf and the branch it attaches to) of a branch that may develop into a new branch

circadian rhythm

repeated process that occurs within a 24-hour period; these patterns continue even without external cues


a class of plant hormones that influences tissue growth


a plant hormone that controls the ripening of fruit


the effect of gravity on plant movement


one of a class of plant hormones that regulates stem length, seed germination, flowering, sexual expression, and leaf and fruit ripening


specialized cell, found in stems and leaves, that makes eating a plant unpleasant, such as sharp crystals within cells


a specific hormone within leaves and stems that triggers a reaction when a plant's surface is wounded

nastic movement

quick movement by plants in reaction to temperature, humidity, or touch; these movements are reversible and repeatable

nutation movement

bending movement by plants in which the plant inclines in a particular direction


disease-causing organism or virus that causes harm to a host organism


plant movement based on light


chemical substance in plants that signals growth and development

plant growth regulator (PGR)

chemical compound that signals growth and development in plants


the aging and decay of plant matter

structural defense

protective plant part, such as a spine or thorn

tactic movement

a plant's response to touch


hairlike outgrowth on a plant shoot that functions to deter herbivores

triple response

the reaction of a plant shoot to avoid an obstacle to growth


a positive or negative response of a plant to a stimulus

turgor movement

a reversible positioning caused by changes in water content in a plant

volatile organic compound (VOC)

organic compound that easily vaporizes; used for communication in plants