Page:The New International Encyclopædia 1st ed. v. 19.djvu/556

* TROPISM. 484 TKOPISM. surfaces to crawl upward : this is negative geot- ropism. The investigation of tropism lias most important bearings on comparative psy- chology and the explanation of instinctive acts. Anemotropism. An orientation of the body with respect to the wind, seen in flies {Bihio ophyra, etc.), which poise in swarms in the air, heading directly toward the wind, all the swarms of a given locality oriented in the same direction like weather-cocks. On the other hand, the locusts move with the wind and head away from it; i.e. they are negatively anemotropic (Wlieeler). Chemotropism. The orientation of an organ- ism by molecules emanating from a centre of diffusion. As the name indicates, it means re- sponse to orders or tastes emanating from food, decaj-ing substances, and other matters. Thus the maggot of the flesh and other flies are posi- tively chemotropic toward certain chemical sub- stances formed in decaying meat and cheese. The female fly is led not so much by a special instinct to these substances as by the stimulus of the rankly smelling meat. As soon, says Loeb, as the fly is seated on the meat chemical stimuli seem to throw into activit}' the muscles concerned in egg-laying and the eggs are deposited on the meat. Negative chemotropism is forcibly illus- trated by the use of the oil of pennyroyal, of to- bacco smoke, or kerosene oil in driving away flies and 'mosquitoes. For the botanical aspect of this tropism, see Chemotropism. Galvanotropism. When certain shrimps, eraj'fish, and salamanders are subjected to the effect of a galvanic current such changes of tension take place in the muscles of the appen- dages that movement toward the anode becomes easier and toward the cathode more difficult. The result is, says Loeb, that if the current is continued long enough all the animals collect at the positive pole. It differs from heliotropism chiefly in that the curves of the current take the place of the rays of light. Geotroplsm. As the result of the action of gravity, plants and certain animals tend to de- scend into or toward the earth. The act of righting in a starfish or sea-urchin, or in a polyp ( sea-anemone ), is the result of geotropic irritability, i.e. of positive geotropism. The effects of geotropism are seen in any moth or butterfly which has just emerged from the pupa- ease. It is restless till it finds a perpendicular surface on which to support itself with its abdo- men hanging down until the wings are fully ex- panded. A fly orients its body head downward, a position assumed by many web-weaving spiders. Geotropism plays a part in the depth-migrations of pelagic animals. See also Geotropism in Plants. Heliotropism or Phototropisii. Originally applied to the property certain plants possess of turning to the light. It has been found that many animals also are oriented toward the light. Loeb claims that in the ease of animals which possess nerves "the movements of orienta- tion toward light are governed by exactly the same external conditions and depend in the same way upon the external form of the body, as in the case of plants which possess no nerves." Many young or larval pelagic animals undergo periodic depth-migrations. They begin to mi- grate vertically upward toward the surface of the ocean in the evening, descending in the morn- ing, but since they never sink beyond a depth of about 1200 feet below, where light docs not penetrate, this fact suggests that the light is the controlling power in their depth-migrations. Loeb finds that the free-swimming animals at the surface of the sea are all permanently or for a short period positively heliotropic. There is a negative as w'ell as a positive heliotropism. If light strikes one side of a positively heliotrop- ic animal, an increase takes place in the tension of those muscles which turn the head to the source of light, while in the negatively heliotrop- ic animal under the influence of one-sided illu- mination a decrease takes place in the tension of the same muscles, with the result that the negatively heliotropic animal is forced to move awa.y from the source of light. The flight of moths toward a light is a case of heliotropism. Being a rapid flyer, the moth gets into the flame before the heat can check it. Paraheliotropism. This is defined by Dar- win as the movement by which some leaves tem- porarily direct their edges to the source of light. Hydrotropism. If a king-crab is placed on shore a few feet from the water's edge it w'ill seek the water, perhaps oriented by the moisture not far away. Ants exhibit a reaction to heat (thermotropism) and also hydrotropism, in the care with which they move the eggs, young, and old larvie and pupae, as the nest becomes too warm or cold, moist or diy, at different times. For its influence on plants, see Hydrotropism. Riieotropism. This is illustrated by the ac- tion of fish in heading up stream and by the slime protozoan in creeping against the current. Stereotropism. This form of tropism is con- tact-irritability, which forces the animal to bring the ventral side of the body in contact with solid bodies. Thus the ventral side of the star- fish is positively stereotropic; in other words, the starfish, oft' the bottom, becomes restless if its ambulacral feet are not in contact with solid bodies.. If earthworms are placed in a transparent closed vessel they appear to be posi- tively stereotropic. As soon, says Loeb, as they reach an angle in the aquarium they remain there, crawling along where the glass can touch them on two sides. This form of tropism plays, says Loeb, a very important part in the processes of pairing and in the formation of organs. The tendency of many animals to creep into cracks and crevices haSj he claims, "nothing to do with self-concealment, but only with the necessity of bringing the body on every side in contact with solid bodies." The eel is positively stereotropic. It is forced to bring every part of its bod.y, as far as possible, in contact with solid bodies. Every segment of the eel's body is stereotropic, and if touched with the finger on one side there result positively stereotropic curvatures toward the finger. There is in such an act no more con- sciousness than in the boring of a root in the sand (Loeb). Stereotropism is called 'thigmot- ropism' by some authors. Another form of trop- ism is traumatotropism. Consult Loeb, Comparative Physioloqij of the Brain and Comparative Psyeholofiy (New York, 1002) ; also Wheeler. "Anemotropism and Other Tropisms in Insects," in Archiv fiir Entwicke- hinnsntechanih der Organismen, vol. viii. (Leip- zig. 1899).