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 animalcule do not react to light. 1 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. 2 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 



., p. 387. 2 ENGELMANN, Botanische Zeitung, 1888. 



