External Factors that Influence Growth 265 
nearly with that given by Yung for tadpoles than with that of 
Vernon for the sea-urchin larve. It is noticeable that in all 
cases the violet stands near the top of the list, but the order of 
red, green, and yellow is not the same in any two cases.’ 
Growth toward the Light; Phototropism 
The turning of plants toward the light is a familiar phenom- 
enon. It is brought about by more rapid growth on the 
shaded side. 
Most animals are free to move, and some of them move toward 
or away from the light. Such turing cannot be called a pro- 
cess of growth, but is due to contraction of the muscles of the 
body. Sense organs, muscles, and nerves are the physiological 
agents in the process. But some animals that are fixed turn 
toward the light, and in these the process seems more nearly to 
approach the condition in plants, although it remains still to 
be determined whether, in reality, the method of turning is the 
same in the two cases. One of the serpulid worms, Spiro- 
graphis, lives in a tough tube formed as a secretion of its 
body. If illuminated from one side the worm turns toward 
the light, causing the tube to bend in this direction. As 
additions are made to the tube, the new part is made in 
the direction of the source of light. Here there cannot be 
said to be a growth of the animal, but only a growth of the 
tube. 
The stolons of the hydroid, Sertularella polyzonias, grow away 
from the light, while the hydranths grow toward the light. In 
another hydroid, Eudendrium, the hydranths also grow toward 
the light. 
In these cases the bending appears to be more nearly like that 
of plants, but, as I have said, the way in which the bending 
occurs has not yet been sufficiently examined. 
‘Yung found the development of Sepia officinalis to be affected by light 
in the following order: violet, blue, yellow and red, green. Fatigat (Compt. 
Rendus, LXXXXIX, December 1879) found for infusoria that violet light 
accelerated and green light retarded the development. 
