1. Plant hormones

2. Hormones Hormone – Gr. « to excite »
Natural chemical messengers that are transported to target cells
Minute concentrations needed to induce a substantial change
Growth regulators

3. Plants respond to stimuli

4. Plant hormones coordinate growth, development and responses to stimuli
Tropisms
Growth response resulting in plant organs curving towards or away from a stimulus
Caused by hormones
Ex. phototropism, thigmotropism, gravitropism

5. Studies on phototropism
Towards or away from light
Differential growth of cells on opposite sides of a shoot or coleoptile
Cells on darker side elongate faster than those on the light side
Experiments by Darwin and son and FW Went
Due to auxin distributions

6. Went experiment