Once the root tip points downward again, the root stops curving and the subsequent growth is straight. After a region responds to a stimulus its orientation is usually permanent. For example, the curvature remains for the life of the root.
In most cases, only the growing regions of the plant are capable of tropisms. There are many such regions in a plant. The tips of stems and roots contain meristems, regions where new growth occurs. Cell divisions in meristems contribute to the elongation of stems and roots, and form new stem branches. Each stem branch is capable of phototropism and gravitropism. Their collective responses help determine the overall shape of the shoot.
By definition, a tropism involves a stimulus that contains some directional information. With gravity, both the direction and strength of the stimulus are uniform. In contrast, the direction of illumination constantly changes, and even heavily shaded plants receive at least some light from all directions. Two conditions must be fulfilled for a tropism to occur in such situations. First, the stimulus must exist in a gradient with respect to the plant. Thus the light is brighter on one side of a shoot, or there is morewater on one side of a root.
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