19.8–19.10: External cues trigger internal responses.

Tendrils of American vetch, Vicia americana, climbing a fence.

A plant’s immobility affects virtually every aspect of its structural design and function. Here we look at some of the evolutionary solutions that enable plants to grow, develop, and reproduce successfully while rooted in a single location. Like animals, for example, plants must respond to their environment as it changes. But whereas animals can move from a problematic, changing environment to one with more suitable conditions, plants instead grow toward or away from various environmental stimuli such as light, gravity, and physical obstacles. In responding to these stimuli, plants use a variety of growth patterns, known as tropisms—such as bending, curving, and twisting. Three of the most common tropisms are phototropism, gravitropism, and thigmotropism.

Phototropism If you have indoor plants, you’ve probably noticed that they always seem to grow toward a window. Even if you turn a plant around, within a few days it will grow to face the light again. This tendency to grow toward a directional light source is phototropism. It occurs when the cells in a plant’s stem grow unevenly, at different rates, in such a way that the stem bends toward the light (FIGURE 19-19). The reason that such a pattern of growth has evolved is not surprising: if a plant can orient itself so that its photosynthesizing cells (in leaves and stems) can intercept more light, the plant can photosynthesize more efficiently and generate more energy for growth and reproduction.

Figure 19.19: Phototropism is plant growth that is influenced by the presence of light.

Figure 19.20: Plant stems grow away from the force of gravity.

It was the study of phototropism that led to the discovery of the plant hormone auxin (which turned out to be a group of similar hormones). When light hits a plant from a particular direction, auxins produced in the plant cells move away from the light source and end up on the opposite, shaded side of the stem. There, the auxins stimulate a slightly greater rate of growth than on the lighter side, where there is less auxin. The elongation of cells on the shaded side of the stem causes the stem to bend toward the light.

Phototropism appears to be a response simply to “light,” but that’s not exactly right. Recall from Chapter 4 that “light” is made up of many different wavelengths. The light is detected by light-absorbing molecules called pigments. The pigments in plants that are responsible for phototropism actually respond only to part of the full spectrum of light—the wavelengths that correspond to blue light.

One special type of phototropism, first described by Leonardo da Vinci, is called heliotropism—growth or movement in response to the position of the sun. “Heliotropic” leaves and flowers, such as the alpine buttercup, change orientation as they track the sun’s movement across the sky each day. Cells in regions of the plant away from the light elongate as potassium ions are pumped into them and the subsequent movement of water into the cells increases turgor pressure (see Section 3-21), which changes the orientation of the flowers or leaves.

Gravitropism Plants’ growth is also guided by gravity. Plant response to gravity, known as gravitropism (also first described by Leonardo da Vinci), is the reason that stems grow upward and roots grow downward. You can observe gravitropism if you take a potted houseplant and tip it on its side. Within a matter of days, the plant will grow upward. It doesn’t matter what direction the pot faces (it can even be suspended upside down); roots will grow downward, in response to the force of gravity—and in the direction in which they are most likely to find water—and stems will grow upward, in the opposite direction, which will reliably put them in position to intercept maximal amounts of light (FIGURE 19-20).

Gravitropism occurs as a result of the uneven distribution of auxins, much as in the case of phototropism. How do the auxins detect the force of gravity? Small bodies that contain starches are present within plant cells. Like marbles in a bottle of fluid, these starch-containing bodies, pulled by gravity, sink toward the bottom of the cell, regardless of the plant’s orientation. Once there, the starch bodies trigger the migration of auxin molecules toward them, and the auxin again causes uneven cell growth so that the stems and roots bend in the right direction.

In a plant turned on its side, the starch-containing bodies in the cells of the stem sink to the cell bottoms in response to gravity and trigger the movement of auxin toward them. The auxin, in turn, causes the stem to grow in an uneven pattern (faster on the side with the auxin), and the stem bends upward. If an apparatus is set up so that the sideways plant is slowly rotated, however, the starch bodies never get a chance to settle on the bottom of the cells (because “the bottom” is constantly changing). Consequently, the auxin never concentrates at the bottom and uneven growth is never stimulated. The plant doesn’t sense which way is up, because “up” keeps changing.

Thigmotropism In thigmotropism, plant growth occurs in response to touch or physical contact with an object (thigmo- derives from the Greek word for “touch”). For example, many climbing plants, such as cucumbers and morning glories, will wrap around structures—other plants, wires, posts, trellises, or anything that might support them as they grow upward. Climbing plants produce tendrils, which are specialized thread-like leaves or stems or branches that wrap around whatever they touch (FIGURE 19-21). In some species, a tendril can grow so fast that it wraps completely around something in less than an hour.

Figure 19.21: Plant growth can be affected by touch or contact with physical objects.

As in the case of gravitropism and phototropism, thigmotropism occurs through uneven growth: cells on one side of a shoot (the side in physical contact with the object) elongate less quickly than cells on the opposite side. This, again, involves auxins. The cells in contact with the object produce auxins, and these auxins are transported to cells not touching the object, where they induce the cells to elongate, causing the tendril to coil around the object.