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Control Systems in Plants Chapter 26 I. Control of Plant Development A. plant hormones 1. chemical messengers within organisms 2. each has a very specific function 3. auxins a. b. promote growth at apical meristems cause growth in different directions 4. gibberellins • promote stem elongation 5. cytokinins a. b. promote cell division prevent premature aging (senescence) 6. ethylene a. b. causes dropping of leaves and fruit (abscission) ripens fruit 7. abscisic acid a. b. promotes seed and bud dormancy closes stomata B. Plant Responses to the Environment 1. tropisms a. b. c. growth toward or away from a variable • positive vs. negative causes growth in different directions phototropism, chemo-, thigmo-, gravi- An example of positive phototropism Fig. 26.11 (a) Negative gravitropism in stem growth. (b) Positive gravitropism in root growth. 2. circadian rhythms a. b. c. activities that occur in 24-hour cycles biological clocks i. internal (genetic) mechanism that maintains circadian rhythms ii. strongly influenced by photoperiod sleep movements Fig. 26.13 Coiling response is an example of thigmotropism. Fig. 26.15 Examples of sleep movements in prayer plants and morning glories. d. photoperiodism i. responses caused by changes in day-length ii. controlled by the pigment phytochrome • acts as a photoreceptor iii. best example is flowering • short-day vs. long-day vs. day-neutral plants Fig. 26.5 Graph of circadian rhythms in morning glories. The influence of photoperiod on flowering in long-day and short-day plants. II. Plant Defenses A. like animals, plants have defenses against disease 1. 2. 3. bacterial, viral, and fungal infections much of their defense is based on genetics chemical attacks against pathogens a. often localized in more vulnerable areas b. phytoalexins, alkaloids, glycosides, etc. B. plants also protect themselves from herbivores 1. 2. structural defenses • spines, thorns, hair layers, tough bark, etc. chemical defenses • toxins, inedible substances, sticky substances, etc. Fig. 26.19 Examples of plant herbivores Fig. 26.19 Plant fungal infection