If you lay a plant on its side, its stems will turn upward and its roots will bend downward. Similarly, if you place a houseplant near a window, the shoot will grow toward the light. Tropism is the bending or turning of an organism in response to an external signal such as light or gravity. Plant stems are positively phototropic, meaning they bend toward the light. Plant stems are also negatively gravitropic, meaning they grow upward against the force of gravity. In contrast, plant roots grow down and away from the light. Thus, roots are positively gravitropic and negatively phototropic.

Observing that grass seedlings bend toward light, Charles Darwin and his son Francis conducted experiments to determine what part of the plant detects light. When they covered just the tip of the young plants, the plants no longer grew toward the light, but when they buried them in fine black sand so that only their tips were exposed, they did. This experiment established the shoot tip as the site where the light is perceived but left open the question of what kind of signal caused the plants to bend (Fig. 31.19).

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Photoreceptors allow plants to detect and respond to light. If a plant is exposed to light from only one side, only the photoreceptors on the illuminated side are activated. Phototropism in plants involves a photoreceptor that absorbs blue wavelengths of light. The absorption of blue light by the photoreceptor is thought to trigger a change in the placement of PIN proteins so that auxin is transported to the shaded side of the stem. As a result, concentrations of auxin are higher on the shaded side and lower on the illuminated side. Because auxin stimulates cell expansion in stems, the shoot grows faster on the shaded side than it does on the illuminated side, causing the plant to bend toward the light (Fig. 31.20).

Phototropism in roots is similar, in that illuminating a root from the side causes auxin to be redistributed toward the shaded side. In this case, auxin is transported to the shaded side within the root cap, and then moves upward in the outermost layer of cells into the zone of cell elongation. However, higher auxin concentrations decrease the rate of cell elongation in roots, causing roots to bend away from light. The reason that auxin redistribution leads to such a different response in roots compared with shoots is that roots and stems differ in their sensitivity to auxin and that in roots high concentrations of auxin trigger the production of the hormone ethylene, which reduces cell elongation.

A similar mechanism orients plants with respect to gravity. Moving a plant to a horizontal position results in the transport of auxin to the lower side. The accumulation of auxin on the lower surface causes stems to bend up and roots to turn down. How is the plant able to detect which way is up and which way is down? Recall that plants store carbohydrates as starch. Starch is a large molecule that has a density greater than water. Starch-filled organelles thus sink to the bottom of the cell. The weight of these organelles pressing on the cytoskeleton or membranes is thought to be the signal that causes auxin to move toward the lower side of the plant. In roots, gravity is a critical source of information regarding orientation. Specialized gravity-sensing cells in the root cap contain large starch-filled organelles known as statoliths (Fig. 31.21).

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