ORGANIC MOVEMENTS 11 



be spoken of as rays. Take the distribution of heat, not 

 by radiation but by conduction, take the diffusion of chemical 

 substances in solutions, and, last not least, take the electric 

 current ; in all these cases we may speak of the existence 

 of " potentials," in the .broadest meaning of the word, and 

 similarly we may speak of the existence of " lines of force." 

 These lines of force, existing in all those processes, not only 

 in galvanism but in diffusion and also in thermic conduc- 

 tion, allow us to speak of directed agents in every case 

 where these lines exist, and in this way the realm of 

 directed agents of the medium becomes very large. In fact, 

 the directed movements we shall speak about, have been 

 found to exist in correspondence with almost all of the 

 directed agents of the medium in this broadest sense. 



A " tropism," then, is a directed movement of a growing 

 part of a plant or hydroid determined by the direction of a 

 directed agent. 



The theory of tropisms l would be a very simple thing 

 if there were nothing but typical cases say of geotropism or 

 of heliotropism, e.g. such cases as the bending of a branch 

 to any source of light, and the invariable bending of roots 

 towards the ground. 



But there are two classes of complications, each of them 

 consisting of two parts. 



There are many cases where the " sense ' of a tropism, 

 that is to say, its being positive or negative, is changed 

 by the intensity say of the light or of the chemical 

 stimulus. An organ that is positive under ordinary 

 conditions begins to bend away from the source of stimu- 



1 In Pfeffer's Pftanzenphysiologie (vol. vii. p. 546) an excellent account of 

 the theory of tropisms will be found. 



