HELIOTROPISM OF ANIMALS 38 
on the form of the body in so far as dorsiventral animals 
move with their median planes in the direction of the rays 
of light, in which position the rays fall upon symmetrically 
situated points of the surface of their bodies at nearly equal 
angles. In this way the fact that a moth flies into a flame 
turns out to be the same mechanical process as that by which 
the axis of the stem of a plant puts itself in the direction of 
the rays of light. In both cases, however—in the fatal 
flight of the moth as well as in the orientation of plants— 
one point remains unexplained, namely: how can the light 
so change the state of the protoplasm as to bring about the 
mechanical effects just mentioned? At present we are not 
able to form a clear idea of this. 
A second condition which has a determining influence 
upon the mechanical effects of light on plants is the refran- 
gibility of the rays. Sachs has shown that it is chiefly the 
more refrangible rays which are able to bring about move- 
ments in plant organisms. We shall see that quite gen- 
erally the more refrangible rays are also more effective 
mechanically in the animal kingdom. 
Thirdly, we shall prove that the orientation of animals as 
well as of plants takes place when the intensity of the light 
remains constant. Very often we observe, for example in our 
eyes, that a change in the intensity of the light acts as a 
stimulus. In addition to these essential considerations of 
the effects of light in the animal kingdom, the following 
factors play a réle, namely: 
Fourthly, light causes the orientation of animals (as well 
as of plants) only within certain limits of intensity. Fifthly, 
temperature influences the movements of orientation in 
animals and plants toward light—which is true for all 
phenomena of stimulation. 
To sum up: The conditions which control the movements 
of animals toward light are identical, point for point, 
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