Chapter 11 Section 2

What are plant responses? All organisms, including plants, respond to stimuli. (Plants respond to touch, gravity, and water) Stimuli (Stimulus)- anything in the environment that effects the behavior of an organism (cause a response).

Tropisms Tropism- response of a plant to an external stimuli. (when a pea plant touches a solid object, it grows faster on one side so it twists around any object it touches.) Positive phototropism- plant grows toward light.

Tropisms Gravitropism- plants response to gravity. The downward growth of plant roots is a positive response to gravity, a stem growing upward is a negative response to gravity. Hormones control the changes in growth that result from tropisms and affect other plant growth.

Plant Hormones Auxin-plant hormone that causes plant stems and leaves to exhibit positive responses to light. (grows toward the light) Abscisic acid is a hormone that keeps seeds from sprouting and buds developing in the winter. Photoperiodism- plant’s response to the number of hours of daylight and darkness it receives.

Plant Hormones Long-day plants- requires less than 10 to 12 hours of darkness to flower (flower in late spring or summer) Short-day plants- need 12 or more hours of darkness to flower (flowers in the late fall or winter) Day-neutral plant- have no specific photoperiod.