Chapter 25 Hormones and Plant Responses

Plant Growth Unlike animals, plant have no true pattern of growth - no pre-determined number of branches and leaves Also unlike animals in adulthood, who stop growing, plants continue to grow by: - adding new needles - adding new wood - produce cones or flowers

Plant Hormones Plant Hormones - chemical substances that control a plant’s division, growth, maturation, and development - can also control plant’s response to the environment Auxins – plant hormone found in the seedling tip or apical meristem that stimulate cell elongation - Stem is stimulated to grow toward the light - Roots cells are inhibited and forced to grow downward - Also responsible for roots growing around objects or obstructions

Plant Hormones Cytokinins – plant hormone that stimulates cell division and growth of lateral buds - may also cause dormant seeds to bud - delays aging of leaves * counteracts effects of auxins - Rate of cell growth in plants is determined by the ratio of auxins to cytokinins

Plant Hormones Gibberellins – hormone known to produce dramatic increases in size of stems and fruit - also responsible for rapid early growth Ethylene – hormone that stimulates the ripening of fruit in the plant Ex: lemons and tomatoes

Plant Responses Tropism – responses of plants to external stimuli - can change patterns and directions of growth - Plants can demonstrate positive (toward stimulus) or negative (away from stimulus) tropisms

Plant Responses 3 kinds of plant tropisms: 1. Phototropism – response or growth toward light 2. Gravitropism – response to the force of gravity – plant roots 3. Thigmotropism – response to touch Ex: growth of vines and climbing plants

Plant Responses Photoperiodism – responsible for timing of seasonal activities such as flowering and growth - Phytochrome plant pigment that causes photoperiodism Ex: - short day plants like poinsettas - long day plants like spinach