A tropism (TROH-piz-uhm) is a response in which a plant grows either toward or away from an environmental stimulus. Growth toward an environmental stimulus is called a positive tropism, and growth away from a stimulus is called a negative tropism. Each kind of tropism is named for its stimulus. For example, plant growth in response to light coming from one direction is called phototropism. Thus, in Figure 31-5, the shoot tips of a plant that grow toward the light source are positively phototropic. Types of tropisms are summarized in Table 31-2.


Phototropism is illustrated by the appearance of the sprouts in Figure 31-5. Scientists have observed that light causes the hormone auxin to move to the shaded side of the shoot. The auxin is thought to cause the cells on the shaded side to elongate more than the cells on the lighted side. As a result, the shoot grows toward the light and exhibits positive phototropism.

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