Biology Semester II

Sections:

Introduction |  Section 1 | Section 2 | Section 3

Section Three:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 | Part 10

Biology: Plant Hormones, Nutrition, and Transport: Part Two Tropisms Charles Darwin and his son Francis studied the familiar reaction of plants curving toward light, a process known as phototropism. They discovered that the tips of the plant curved first and that the curve extended gradually down the stem. By covering the tips with foil, Charles and Francis Darwin prevented the plant from curving. They concluded that some factor, which they did not identify, was transmitted from the tip of the plant to the lower regions, causing the plant to bend. This factor is known now to be auxin, a plant growth hormone. Phototropism in radish seedlings. The light source is on the right side of the photograph. Monocot stems have a protective sheath, called the coleoptile. They curve toward light due to an unequal distribution of a plant growth hormone, auxin. The higher concentrations of this chemical on the darker side of the plant cause uneven elongation of cells, resulting in the curving of the plant stem toward light. Phototropism in the coleoptile of a monocot. With a series of experiments in 1926, Dutch plant physiologist Frits Went isolated auxin. He proved that it was the “influence” discussed by the Darwins. Auxin moves to the darker side of the plant and causes the cells there to grow larger than corresponding cells on the lighter side of the plant. This produces a curving of the plant stem tip toward the light. Gravitropism (formerly known as geotropism) is plant response to gravity. How do roots “know” which direction is down? How do stems “know” to grow upward, even when they are just emerging from a seed that is below the ground? Roots of plants show positive gravitropism. Shoots exhibit negative gravitropism. Gravitropism was once thought to result from gravity’s influence on auxin concentration. However, several recent ideas suggest that auxin combines with the movement of calcium ions to cause root gravitropism. Timothy Caspar and colleagues at Michigan State University contend that the movement of starch granules within cells explains why roots grow downward. A competing hypothesis, proposed by Randy Wayne and colleagues from Cornell University, proposes that cell membrane proteins may be responsible for the detection of gravity. Gravitropism in a coleoptile. Gravitropism of a seed (top) and stem (bottom). By an as yet undetermined mechanism, plant stems, shown in a, curve upward, while plant roots curve downward. Thigmotropism is plant response to contact with a solid object. Tendrils of vines, such as the passionflower, wrap around objects and allow the vine to grow upward. The stems of some plants, such as the morning glory, intertwine or braid to allow upward growth. Specialized epidermal cells detect contact, causing the plant to grow in a specific direction. The combined effects of the plant hormones, auxin and ethylene, are thought to cause thigmotropism. Thigmotropism in a tendril of a passionflower as it grows to wrap around a metal rod. Nastic movements, such as nyctinasty, result from several types of stimuli, including light and touch. Legumes turn their leaves in response to day and night conditions. Mimosa, also known as the sensitive plant, closes up its leaves when touched. Thigmonasty in the sensitive plant Mimosa. Now go on to the next part.

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