Phototropism and gravitropism lead to the bending of shoots or roots toward or away from a stimulus (light or gravity, respectively). Such bending is brought about by:
a. increased growth over the general surface of the root or shoot.
b. differential growth on opposite sides of the root or shoot.
c. muscular contractions, leading to a curvature in the root or shoot.
d. loss of water into the atmosphere.
e. all of the above.