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Control of Growth and Development
causing the cells there to grow at the fastest rate. Thus, the plant growing indoors is forced to bend toward the window because light coming from that direction induces a redistribution of auxin in the stem, resulting in uneven growth. Although the underlying principle of phototropism seems uncomplicated, the details of how the process takes place are not fully understood. Plant physiologists are currently studying how IAA moves from brightly lit to shaded cells; and the complex biochemistry of cell growth, including cell wall expansion, is still under investigation. Growth of stem internodes is promoted by another hormone, named gibberellin after the fungus Gibberella in which it was first discovered. The action of gibberellin on internode cells is also related to light intensity. In full sun the hormone’s effect on growth is somewhat restrained. Thus, while gibberellin promotes sufficient internode elongation to space the leaves, the structural stability of a squat growth form is maintained. In low light intensities, however, gibberellin becomes more active, causing internodes to stretch. By so doing the upper leaves are elevated to a position where they are better able to locate light, especially in competitive situations with surrounding plants. Interestingly, shade-loving species, being fully adapted to their preferred habitats, show no such reactions to low light intensities. It appears to be a state of desperation that makes dark-grown, etiolated plants direct all of their energies into internode elongation as they search for a vestige of light. Gibberellin’s unbridled stimulation brings about such growth. But rarely does the response result in a successful outcome as reserve food supplies are soon exhausted. RESPONSES TO GRAVITY AND TOUCH
When roots and shoots grow in opposite directions they reflect contrary responses to Earth’s gravitational field. Geotropism (Greek: ge, “earth”), or gravitropism, are the names given the physiological process. Most roots are positively geotropic; that is, they grow in the direction of gravity. Stems, for the most part, are negatively geotropic, growing opposite to the gravitational force. When rhizomes, stolons, and some roots grow horizontally, they display diageotropism (Greek: dia-, “across”). And branches from roots and stems, developing at an angle from the vertical plant axis, are plagiotropic (Greek: plagios, “oblique”). When a stem is placed on its side (perhaps a potted plant is accidentally tipped over), the apex soon returns to its normal direction of growth. Under the influence of gravity, auxin collects in the lower side of the stem where it stimulates the cells to grow more rapidly than those across the top.
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