TROPISMS AND RELATED PHENOMENA 149 



to grow down vertically, the geotropic reaction is not so precise as in 

 the stem. The latter is negatively, the former, positively, geotropic. 

 In free-moving animals, geotropism is not so rare; Cucumaria cucumis, 

 a Holothurian, possesses five rows of feet with which it can creep on ver- 

 tical surfaces. If the animal be put on a vertical glass plate, it will 

 creep vertically upward. When the plate is turned very slowly around 

 a horizontal axis, the animal remains quiet during the rotation, but as 

 soon as the plate is fixed, the Cucumaria again creeps vertically up- 

 ward. This occurs also in the dark room. We will call such animals 

 that are compelled to creep vertically upward, negatively geotropic. 



Many marine animals and many insects show this reaction. If 

 free-swimming aquatic animals which show a tendency to gather at 

 the top or the bottom of a vessel are used for experimentation, the ex- 

 perimenter must be careful not to mistake the passive sinking or rising 

 of such forms for geotropic reaction. Ostwald has called attention 

 to the fact that with increasing temperature the internal friction of the 

 water diminishes rapidly, which necessitates that organisms which 

 float at the surface at a lower temperature must either sink down at a 

 higher temperature, or are unable to work upward, on account of the 

 diminished internal friction of the water. In addition, the resistance 

 due to the shape of the animal plays a role in these phenomena.* 



The attempt which many animals make to keep the axes of their 

 eyes as nearly as possible in their normal position in space when the 

 body is put into an abnormal position, is a common reaction which 

 seems to be determined by gravitation. In Crustaceans the eyestalks 

 form a small angle with the horizontal plane when the animal is in its 

 normal position. When turned on one side, however, so that the right 

 side is directly downward, the eyes no longer keep their symmetrical 

 position in regard to the plane of symmetry of the animal, but the right 

 eye is raised, the left lowered. f It looks as if the eyes had a tendency 

 to keep their former normal position in space ; just as the root or stem 

 of a geotropic plant tries to keep its orientation toward the center of 

 the earth. This reaction of the eyes also exists in vertebrates, and 

 can be nicely demonstrated in fishes, lizards, birds, or rabbits. In 

 frogs the eyes do not show the so-called compensatory motions, but 

 the head as a whole tries to keep its normal orientation toward the 

 horizon, when the body of the animal is put into an abnormal posi- 

 tioQ. These reactions exist universally, but in such forms as possess a 

 powerful associative memory the reaction is liable to be interfered with. 



* Wolfgang Ostwald, Zoologische Jahrbucher, Vol. 1 8, p. I, 1903. 



t Clark, Jour, of Physiology, Vol. 19, p. 327, 1896. E. P. Lyon, Am. Jour, Physiology, 

 Vol. 3, p. 86, 1899. 



