II 



FURTHER INVESTIGATIONS ON THE HELIOTROPISM 

 OF ANIMALS AND ITS IDENTITY WITH THE HELI- 

 OTROPISM OF PLANTS 1 



IN a former paper I showed that the dependence of 

 animal movement upon light is identical with that of plants 

 on the same source of stimulation. 2 I showed that the law 

 put forward by Sachs for the heliotropism of plants, 

 namely, that the direction of the rays of light deter- 

 mines the orientation, holds good also for animals. Free- 

 moving animals are compelled to execute their progressive 

 movements in the direction of the rays of light, as is the case 

 with the swarm-spores of certain Algse. It was further 

 proved that the more refrangible rays of the visible spectrum 

 are the rays that are solely, or at least chiefly, effective in 

 bringing about the movements of -these animals ; as is the 

 case in the heliotropic movement of plants. After I had 

 proved the identity of this relationship point for point, I 

 believed it permissible to designate these reactions of ani- 

 mals by the same term as that used for the identical reactions 

 of plants, namely, "heliotropism." 



At that time, however, I had proved this identity only in 

 the case of free-moving animals. The task still remained to 

 ascertain and investigate the influence of light upon the 

 orientation of sessile animals, and to decide whether the 

 influence of the light is in this case also similar to that upon 

 sessile plants. It is known that in plants the direction of the 



iPflilgers Archiv, Vol. XL VII (1890), p. 391. 



2 Part I, p. 1. See also LOEB, Sitzungsberichte der Wiirzburger physikalisch- 

 medicinischen Gesellschaft (1888) ; GROOM UND LOEB, Biologisches Centralblatt, Vol. X 

 (1890J. 



