External Factors that Influence Growth 265 



nearly with that given by Yung for tadpoles than with that of 

 Vernon for the sea-urchin larvae. 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 famihar 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 turning 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 reahty, the method of turning is the 

 same in the two cases. One of the serpulid worms, Spiro- 

 graphis, hves 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 hght, while the hydranths grow tow^ard the Hght. In 

 another hydroid, Eudendrium, the hydranths also grow toward 

 the hght. 



In these cases the bending appears to be more nearly hke 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. 



