Greg Vollmer, age 13, of Annapolis, Md., for his question:
CAN YOU EXPLAIN TROPISM?
Tropism is the fixed, automatic, inherited movements in response to particular stimuli. Movement toward the source of stimulation is known as positive tropism. Movement away from the source is known as negative tropism.
In pioneering work in plant tropism, an English naturalist named Charles Darwin demonstrated in 1880 that growing tips of plants bend toward a light source. This phenomenon is known as phototropism. Darwin also observed that some shade plants turn away from bright light by a negative form of phototropism.
The turning of the plant is due to the action of a hormone called auxin, which causes elongation. On the side of a plant facing the light the auxin is inactive; only the side away from the light elongates. Hence the plant tends to bend toward the light.
In animal tropism the response is due to a chemical stimulus called chemotropism. Flies and other insects are attracted to certain odors emanating from the chemical decomposition of meat or other suitable mediums and are stimulated to lay their eggs there.
Other commonly observed tropisms include galvanotropism or electrotropism, movement in response to an electrical current; theotropism, or orientation in response to the direction of a current of water; anemotropism, or movement with respect to the wind; and thermotropism, or movement in relation to unequal conditions of temperature.
Thigmotropism in many lower animals enables them to respond to specific crevices and to differentiate between rough and smooth areas.
The term "tropism" was formerly applied only to the responsive movements of fixed organisms such as rooted plants and attached animals; orientative movements among free swimming or freely locomotive animals being known by the term "taxis." Although today the terms are sometimes used interchangeably, "taxis" is preferred in describing movements of swimming bodies.
When a seed germinates, the young root turns downward regardless of the way in which the seed is planted. This bending, known as positive geotropism, enables a plant to anchor itself in the soil. The young stem, which turns upward away from the earth, is said to be negatively geotropic.
Vines have to depend for their support on other plants or surfaces, and the tendency of the vine to respond to touch or contact with such supports is known as thigmotropism.
In 1975 scientists observed that vine tips creep along the ground toward vertical objects by responding to the stimulus of the darkest sector of the nearby horizon. This response was termed skototropism, or "growing toward darkness."
An individual organism may exhibit a positive or negative tropism to the same stimulus at different times, depending on the strength of the stimulation and the internal physiological conditions of the organism. Among the higher animals, learned rather than stereotyped responses become increasingly prominent.