1. Plant Responses to Stimuli

2. Tropism – movement toward or away from a stimuli
Phototropism – growth toward light
Gravitropism – growth against or with gravity
Thigmotropism – growth toward touch

3. Plant Hormones – Chemicals that control the growth, development, and responses to stimuli
Auxin
phototropism
Gibberelin
growth
Cytokinin
cell division
Abscisic Acid
environment stress
Ethylene
ripening

4. Photoperiodisms – the physiological reaction to the length of day or night. They are sensitive to UV light.
Long-day plants: will only flower if the night is short enough. They will flower in the late Spring early Summer.
Short-day plants: Need a long night to flower.
Day-neutral plants: These are sensitive to temperature

5. Biological Clocks
Circadian rhythms – (“circa” approximately “dies” day) physiological cycles based on 24 hour periods.
The leaves of plants will rise and fall every 24 hours, even in constant light or darkness.

6. Other cycles Ultradian cycle - Many in one day. Seconds to minutes
Infradian cycle – longer than a day – months and years

7. Time Cycles Annuals – complete life cycle in one year (zinnia)
Biennials –
life cycle in 2 years (foxglove)
Perennials –
live many years (tulips)