1. Plant Hormones How do we know they exist? Why do the shoots grow up, but the roots grow down?

2. Plant Hormones Phototropism experiments by Charles and Francis Darwin:

3. Plant Hormones Boysen-Jensen’s experiments: Mechanisms of bending:

4. Plant Hormones Frits Went discovered the hormone Auxin. Concentration of auxins in different parts of roots and shoots determines growth rates of the cells… Since distribution of auxins is affected by both light and gravity, auxins are involved in plants’ phototropic and gravotropic growth patterns.

5. Plant Hormones Major plant hormones and their action: Abscisic acidclosing of stomata; seed dormancy Auxinselongation of shoots and roots, gravotropism, phototropism Cytokininspromotion of sprouting of lateral buds Ethyleneripening of fruit Gibberellinsgermination of seeds and sprouting of buds; elongation of stems; stimulation of flowering; development of fruit

6. Plant Hormones Hormones can work with each other to regulate various aspects of plant biology. Here, auxin and cytokinin work together in regulating lateral bud formation… the sprouting of lateral buds depends on the ratio of cytokinin to auxin…

7. Plant Hormones Plants use starch-filled organelles called statoliths to detect gravity… this is very similar to how jellyfish detect gravity, and in some ways, it is even similar to how humans keep themselves balanced…

8. Plant Hormones A hormone called florigen both stimulates and inhibits flowering. Since flowering is sometimes a function of day length, pigments called phytochromes are involved in “measuring” day length. The ratio of different forms of this pigments change as a function sunlight exposure and can thus be used by the plants to set their “internal biological clocks”.

9. Chemical Communication in Plants

10. Plant Communication The Venus flytrap catches flies as a source of nitrogen. Insects are attracted to the nectar in the trap, but when they trigger sensory “haris” within the trap, they are captured… What is the mechanism?

11. Plant Communication The mechanism is based on “irreversible, differential growth”. Triggering of the hairs creates an electrical potential that pumps H + into outer cell walls. The acidic condition created by the H + induces enzymes to loosen the cellulose fibers of the walls, causing water to rush in… the “growing” or swelling of the outer cells then causes the rapid closing of the “trap”…