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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.

Moreover we know from experiments on heliotropism, thermotropism, etc., that even intangible stimuli such as rays of light, etc., impinging unsymmetrically on these delicate cells cause alterations in their behaviour—e.g. arrest or acceleration of growth.