A plant tropism is movement in response to external stimulation. A plant that requires bright sunlight will bend toward the greatest source of light available if the ambient light is not enough to support its photosynthetic needs. Some plant tropisms are related to growth -- the plant responds to a stimulus by growing in a particular way. Other tropisms are unrelated to growth and happen faster, sometimes as quickly as in a second or two.
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Phototropism is a growth response to sunlight. A plant that bends toward sunlight exhibits a positive tropism, and one that bends away from it exhibits a negative tropism. Light is the cause of phototropism in plants, in which a plant hormone called auxin is inactivated by sunlight on the side of the plant facing the light. On the other side of the plant, auxin causes the cells in the stem to elongate, forcing the plant to bend toward the light.
The ability of some plants to track the sun is unrelated to growth. Certain plants turn their leaves or flowers perpendicular to the sun and follow it throughout the day, tracking from east to west. Pressure changes in cells located in the stems of leaves or just below flower heads cause the movement. Some plants that track the sun become attuned to the day length and move their leaves or flowers to face east as dawn nears.
The response of a plant to gravity is gravitropism. The root of a germinating seed will turn and grow downward toward the Earth's gravity in positive response, while the stem of the plant turns in the exact opposite direction. Gravity is the cause or stimulus of this tropism and calcium migration is the cause of the movement. Calcium moves to the lower surfaces of roots and the upper surfaces of shoots. Elongation occurs in the cells with the lowest concentration of calcium, thus a root bends down toward gravity and a shoot bends upward away from it.
Thigmotropism is a non-growth response to touch. Pressure sensitivity of specialised cells cause the plant to turn or twine around another object, sometimes another plant. The cells not receiving the pressure elongate, causing the plant to turn or twine. Some ivy plants turn fast enough to be observed, taking about an hour to make entire turn. The vine stem itself may turn, or the vine will produce tendrils that turn around the object and support the plant.
A plant may respond to darkness by growing toward it. On the ground, ivy will detect the darkest shape on its horizon and grow in its direction. This is skototropism. Upon reaching the dark object or shape, thigmotropism takes over and the ivy climbs the object. Skototropism is controlled by the plant hormone auxin and is a positive growth response to darkness.
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