24 OF VITAL MOTION. 



are always more perfectly developed than the root and 

 rootlets. 



In all these changes light seems to determine the 

 formation of additional nuclei or centres of growth, 

 and to be the prime mover of the act of nutrition. As 

 heat stands related to development, so light appears 

 to be connected with genesis : how and by what means 

 is not easy to decide, but still we may obtain much 

 information if we study the fact attentively. Light, 

 it may be said, involves a certain clearness of the 

 atmosphere and absence from clouds, which is favour- 

 able to evaporation and radiation; and under these 

 circumstances it may be supposed that the organic 

 fluid in which the nuclei originate tends rapidly to 

 give off its more watery portions. This process pro- 

 ceeds until at last the remaining fluid is no longer 

 able to retain the organic matter in solution, and 

 hence perhaps the appearance of the solid particles. 

 In this way the formation of nuclei in the organic 

 fluid, and of crystals in an ordinary chemical solution, 

 are both favoured by the operation of light, for a state 

 of atmosphere is involved in the presence of this 

 agent which would bring about the necessary evapo- 

 ration of the solvent. 



I am not prepared to deny that light has a special 

 action upon growth, for it is directly concerned in the 

 changes of the photographic process ; but at the same 

 time it is to be borne in mind that this agent is cor- 



