HELIOTROPISM OF ANIMALS 15 
orientation, which alone interest us here, there are also cer- 
tain indirect effects on the orientation of low forms of life. 
These were also first observed by Engelmann. When the 
supply of oxygen is cut off from certain chlorophyll-bearing 
organisms, they remain in that part of the spectrum in which 
assimilation takes place. In water with its normal amount 
of oxygen, as Engelmann found, Stentor viridis, Bursaria, 
and the green slipper animalculz do not react to light.’ If, 
however, the supply of oxygen from without is interfered 
with, “the insufficient supply can be compensated for by a 
production of oxygen by the chlorophyll granules within the 
mesoplasm.” Under these conditions the animals return to 
the light side of the drop when they accidentally get into the 
shady part. When the animals are brought into a micro- 
spectrum, they collect in those regions which promote assimi- 
lation. The opposite effect takes place, however, when the 
supply of oxygen from without exceeds the normal. When 
Engelmann passed a stream of pure oxygen through the 
water, the animals moved from the lighted into the shaded 
part of the drop. 
Such an indirect orientation toward light as is determined 
by assimilation is shown also in the behavior of the purple 
bacteria.” These, as Engelmann found, collect in those 
regions of the spectrum which are. most absorbed by the 
coloring matter of the bacteria. 
These are the most important facts which up to this time 
are known concerning the influence of light on the orienta- 
tion of animals. Thus far only the observations made on 
Infusoria are sufficient to warrant the conclusion that ani- 
mal movements depend on light in the same way as_ the 
movements of plants. In the rest of the animal kingdom 
either the facts necessary for this conclusion are lacking, or 
false statements and conceptions are prevalent. So far as 
1 Tbid., p. 387. 2 ENGELMANN, Botanische Zeitung, 1888. 
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