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Several kinds of tropisms are recognized, such as phototropism or heliotropism, reaction to light; thermotropism, reaction to heat; electrotropism or galvanotropism, to electric current; geotropism, to gravity; chemotropism, to a chemical; rheotropism, to current; thigmotropism or stereotropism, to contact; and chromotropism, to color.

Here, too, must be classed also all the innumerable phenomena of Heliotropism, Geotropism, Rheotropism, Chemotropism, and other tropisms, in which the sun, or the earth, or currents, or chemical stimuli so affect a form of life—plant, alga, or spore—that it disposes its own movements or the arrangements of its parts accordingly, turning towards, or away from, or in an oblique direction to the source of stimulus, or otherwise behaving in some definite manner which could not have been deduced or predicted from the direct effects of the stimulating factors.