Tropisms

Gravitropism is the growth response to gravity. Shoots grow up, roots grow down. Auxin may play a role here. Roots tend to grow towards the pull of gravity, and shoots grow against it. Auxin is involved in gravitropism by way of causing asymmetric cell elongation.

These videos from the Plants in Motion web site illustrate gravitropism in Arabadopsis and root gravitropism

This video (No Audio) describes gravitropism.

Phototropism is a growth response toward light caused by auxin stimulation of cells not exposed to light.

This animation (Audio - Important) describes phototropism.

These videos from the Plants in Motion web site illustrate phototropism of tomato, and corn.

Thigmotropism is unequal growth triggered by physical contact. It is found in climbing vines and tendrils which wrap around supports. Cells on the contact side elongate, causing stems to curl. Auxin and ethylene may be involved.

Effects of Mechanical Stress

Prevailing winds and grazing animals can inhibit stem elongation and plant growth. Simply shaking a plant daily for a brief period will inhibit growth.

REVIEW: Light of _____ is the strongest stimulus for phototropism.
Answer blue wavelengths