. Biology; the story of living things. 368 THE MAINTENANCE OF THE INDIVIDUAL swim backwards, while in the former case it can only crawl forward. In both cases the change in position is caused by the action of the current on the flexor and extensor muscles, which in one case are contracted and the other case extended, thus causing the animal to assume the po- sitions mentioned. Experiments such as these give rise to the theory of tropisms, which is simply another term for a series of responses of an organism to the various factors of its environment. Tropisms may be briefly classified as photot
. Biology; the story of living things. 368 THE MAINTENANCE OF THE INDIVIDUAL swim backwards, while in the former case it can only crawl forward. In both cases the change in position is caused by the action of the current on the flexor and extensor muscles, which in one case are contracted and the other case extended, thus causing the animal to assume the po- sitions mentioned. Experiments such as these give rise to the theory of tropisms, which is simply another term for a series of responses of an organism to the various factors of its environment. Tropisms may be briefly classified as phototro- pisms, or responses to light; geo- tropisms, or responses to gravity ; hydrotropisms, or responses to water; chcmotropisms, or responses to chemical substances ; thermotro- pisms, or responses to temperature changes; galvanotropisms, or re- sponses to electricity; thigmotro- pisms, or responses to contact; rheotropisms, or responses to water currents; and aneinotropisms, or responses to air currents. A tropism is a kind of directional urge. It represents a condition within an organism, resulting from the interaction between its struc- ture (nervous) and the stimuli of the environment. Loeb explained tropisms as specific irritabilities or sensitivities to stimuli at the surface of the body, and in terms of body symmetry, since corre- sponding parts on two sides of the body would show the same sen- sitivities. Noncorresponding parts, according to this theory, would show unequal sensitivities, resulting in directive movements. Loeb explained his famous example of the reversal of tropisms in a caterpillar by showing that the caterpillar moves toward light when hungry and is irresponsive to light when satisfied. The result is most useful to a caterpillar, because as it leaves its nest when hun- gry, it is near the surface of the ground and is drawn by light to the tips of the branches where young edible leaves are sprouting, returning to the lower branches when nonresponsive to
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