19.4–19.7: Hormones regulate growth and development.

A weightlifter hoists the winning marrow (120 pounds) at a giant vegetable competition.

Nature versus nurture. The complex interactions between nature and nurture that influence phenotypes are just as important to plants as they are to animals. Consider a simple experiment with the arrowleaf plant (Sagittaria sagittifolia) as an example. First, from a single plant, take three cuttings. These are sections of the plant containing a part of the stem that includes a meristem. Cuttings can be used to produce a new plant, genetically identical to the original. Plant each cutting in a different habitat: one in deep water, the second in shallow water, and the third on land.

Each new plant has the exact same set of genes; they are like identical triplets. But in the deep water, the plant will grow long, ribbon-like leaves. In the shallow water, the plant will develop large, round leaves resembling lily pads. And on land, the plant will form the arrow-shaped leaves from which it gets its name (FIGURE 19-8).

Figure 19.8: Environment affects the growth patterns of the arrowleaf plant. The differences in growth forms are mediated by hormones.

Figure 19.9: The location and function of five types of plant hormones.

The arrowleaf plant is not “hard-wired” to produce any one type of leaf shape. Rather, the physical form of the plant depends on the environment in which it grows. How does the plant change its growth patterns to respond to different environments? Through the use of hormones.

Plant hormones are chemical signals that enable plants to respond quickly and appropriately to changing environmental variables (such as amount of moisture, amount and direction of sunlight, and temperature). These chemicals convey information about the physiological state of the plant’s tissues or the environment in which the plant finds itself and then regulate some aspect of the metabolism of the plant’s cells. Sometimes, a hormone may stimulate a certain response—such as growth—in the target cell. Other times, the hormone may suppress an action in the target cell.

The hormones in humans and other animals are generally produced in specific glands or tissues and then transported (commonly in the bloodstream) to different locations to exert their influence. In contrast, many plant hormones are produced in numerous places throughout the plant body and may have their effects in those same places or may be transported to another part of the plant before taking effect. As we will see, the effects of plant hormones are usually predictable and easily observed.

There are many plant hormones, many of which fall into one of five major groups: gibberellins, auxins, ethylene, abscisic acid, and cytokinins. FIGURE 19-9 provides a brief summary of the functions of these hormones. We explore them in greater detail in the next three sections.