Cracking the AP Biology Exam




Plants need light. Notice that all the plants in your house tip toward the windows. This movement toward the light is known as phototropism. As you also know, plants generally grow up and down: The branches grow upward, while the roots grow downward into the soil, seeking water. This tendency to grow toward or away from the earth is called gravitropism. All of these tropisms are examples of behavior in plants.

A tropism is a turning response to a stimulus.

There are three basic tropisms in plants. They’re easy to remember because their prefixes indicate the stimuli to which plants react:

1. Phototropism refers to how plants respond to sunlight. For example, plants always bend toward light.

2. Gravitropism refers to how plants respond to gravity. Stems exhibit negative gravitropism (i.e., they grow up, away from the pull of gravity), whereas roots exhibit positive gravitropism (i.e., they grow downward into the earth).

3. Thigmotropism refers to how plants respond to touch. For example, ivy grows around a post or trellis.

These responses are initiated by hormones. The major plant hormones you need to know belong to a class called auxins. Auxins serve many functions in plants. They can promote growth on one side of the plant. For example, in phototropism, the side of the plant that faces away from the sunlight grows faster, thanks to the plant’s auxins, making the plant bend toward the light.

Generally speaking, auxins are in the tip of the plant, because this is where most growth occurs. Auxins are also involved in cell elongation and fruit development.

Other plant hormones that regulate the growth and development of plants are gibberellins, cytokinins, ethylenes, and abscisic acid. Here’s a summary of the functions of plant hormones: