152 PHYSICAL CHEMISTRY 



Rhumbler (1898) made a model of a contractile vacuole in 

 the following manner. A drop of chloroform (not freed from 

 alcohol, and that had become acid) was immersed under water. 

 A cloud of water droplets appeared within the chloroform! and 

 fused to one drop which burst. 



The word tropism if used in strict accordance with its deriva- 

 tion would be confined to the multicellular and syncytial plants, 

 but it has been extended to unicellular plants and to animals as 

 well. For instance, the roots of plants are influenced by gravity. 

 If moved out of the normal relation to gravity the tip bends 

 back until the normal relation is regained. Starch grains in cer- 

 tain cells near the root tip are acted on by gravity and seem to 

 act as otoliths stimulating the cells when the root is placed in 

 an abnormal position. 



Engelmann (1882) showed that the swarm spores of many 

 green plants are influenced in their movements by external agents. 

 He called these reactions to the environment tropisms, although 

 the reaction is not a growing into a new position, but a turning 

 movement or change in the direction of swimming. 



Loeb (1889) bas attempted to show that the tropisms of ani- 

 mals are the same as those of plants. This is largely a philo- 

 sophical question, since the tropisms of plants are very 

 imperfectly understood, and there is no consensus of opinion 

 as to the dividing line between plants and animals. 



Loeb distinguishes tropisms and sensibility to change, defining 

 tropisms as the direct action of the environment on the motor 

 organs, producing a direct and correct response. It should be 

 added, however, that he also considers the reactions of certain 

 insects to light as tropisms but cannot assume that the light acts 

 directly on the muscles, the distinguishing characteristic being 

 that the movements are forced. Even the behavior of vertebrates 

 is sometimes described as tropism. Selachians do not seem to 

 possess much intelligence since they butt their heads to pieces 

 on the glass of aquaria until they die. All of their reactions are 

 not simple, however. The author has often observed the rays 

 of Mission Bay, San Diego, Cal., escape after being discovered. 

 The ray moves along the bottom leaving a trail of muddy water, 

 then doubles under this curtain and finally darts out at a new 

 place. 



