Plants require light for growth and survival, so it makes sense that plant shoots would have mechanisms to maximize their exposure to light. Two different kinds of phenomena help plants do this: phototropism, which is growth toward or away from light, and gravitropism, which is growth toward or away from the force of gravity. In shoots, phototropism and gravitropism are mediated in part by a hormone, called auxin, which travels asymmetrically in the shoot and stimulates cell elongation. In an accompanying animation, we examine the movements of auxin along the shoot and the subsequent growth of the seedling during phototropism and gravitropism.
In another animation, we study the kinds of experiments—similar to those first performed by Charles Darwin and his son Francis—that determined the location of the light sensor in shoots. These experiments were performed on canary grass seedlings, which have a sheath, called a coleoptile, that grows in the direction of a light source.
In gravitropism, the force of gravity induces the coleoptile to transport more auxin along its lower side. The auxin causes these lower cells to elongate, forcing the coleoptile to grow upward.
In phototropism, light induces the coleoptile to transport auxin toward the shaded side of the coleoptile. Auxin causes cell walls to loosen, allowing cells on the shaded side to elongate. The elongation of cells on the shaded side bends the coleoptile toward the light. Bending occurs a few millimeters from the tip.
Choice: opaque cap
Choice: clear cap, opaque sheath, or clear sheath
Charles Darwin and his son Francis performed these types of experiments and determined that the tip of the coleoptile is the light-sensitive region. In blindfolding the tip, the experimenters made the seedling unresponsive to the direction of light. However, blindfolds on other regions did not produce this effect.
According to experiments by the Darwins, the light sensor that plays a role in phototropism lies within the tip of a plant shoot. This is also where auxin is first manufactured in the plant before the hormone is transported from the shoot down the plant. During phototropism, auxin is transported preferentially along the shaded side of the stem, causing the cells on the shaded side to respond by elongating. The elongation of these cells curves the stem toward light.
In negative gravitropism, the stem responds to a gravitational cue, rather than a light cue. In this case, it appears that auxin also mediates the final response, triggering cells on the lower side of the stem to elongate and thereby forcing the plant to grow upward.
Plant roots have the opposite reactions to light and gravity. In the case of roots, they grow away from a light source (negative phototropism) and with the force of gravity (positive gravitropism). These responses ensure that plant roots grow downward, into the soil, where they can find water and minerals.