II 
FURTHER INVESTIGATIONS ON THE HELIOTROPISM 
OF ANIMALS AND ITS IDENTITY WITH THE HELI- 
OTROPISM OF PLANTS! 
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.” 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 Algez. 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 
1 Pfliigers Archiv, Vol. XLVII (1890), p. 391. 
2PartI,p.1. See also Logs, Sitzungsberichte der Wirzburger physikalisch- 
medicinischen Gesellschaft (1888) ; GRooM UND LOEB, Biologisches Centralblatt, Vol. X 
(1890). 
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