The environment has a great effect on a plant's shape. Tropism is a growth response that results in the bending of plants toward or away from a stimulus. Three of the stimuli that encourage tropisms and a change in the plant's body shape are light (phototropism), gravity (gravitropism), and touch (thigmotropism).

Phototropism (directional growth due to light) could result from auxin stimulating cell elongation on the darker side of a stem (found in the classic example of grass coleoptiles) or some other chemical messenger inhibiting elongation on the lighter side (found in the newer studies conducted on the stems of sunflowers and radishes); many researchers agree that the shoot tip is the site of the photoreception that triggers the growth response. Photoreceptors are pigment molecules called cryptochromes, which are most sensitive to blue light.

Gravitrophism (directional growth due to gravity) begins as soon as a seed germinates, ensuring...