1] UPON THE RATE OF GROWTH 427 



There is thus a considerable body of evidence that a not too 

 intense light accelerates the growth of animals in general. 



To sum up, we find that diffuse light accelerates growth in 

 the following organisms : Mistletoe seedlings, Coleochaeta, etio- 

 lated Spirogyra, diatoms, germinating spores of many ferns, 

 mosses, hepatics, and Vaucheria, germinating seeds of Viscum 

 and small-seeded grasses, tadpoles, embryo snails, trout, and, 

 perhaps, young kittens; in brief, of a parasitic seedling, of 

 algse, of germinating spores in many higher cryptogams, of a 

 few germinating seeds, and of some animals. The collection 

 seems like a heterogeneous one ; yet omitting germination 

 phenomena, the case of the parasitic mistletoe, and the doubtful 

 cat, all the organisms concerned are aquatic. The reason why 

 the growth of aquatic organisms is not restrained by light may 

 be that with them light does not produce increased trans- 

 piration. 



3. The Effective Rays. --As we have seen in the First Part 

 the various rays of which white light is composed affect proto- 

 plasm diversely. The question now arises, What part do the 

 separate kinds of rays play in the retarding and accelerating 

 effect of light on growth what are the rays upon which these 

 effects especially depend? 



At the outset it must be stated that a large part of the recorded observa- 

 tions upon this subject is worthless because the methods were not quantita- 

 tive. Let us suppose we have a white light of known intensity which in- 

 hibits growth, and that we desire to know which of the component rays are 

 most effective in imbibition. The different component rays should have the 

 same intensity as they have in the given white light. For a more effective 

 ray of weak intensity will produce a smaller result than a less effective ray 

 of great intensity; because not only quality but intensity of the light deter- 

 mines its effect. Now, insufficient care has been taken to measure, by the 

 methods given in Chapter VII, the intensity of the colored light employed; 

 consequently it is little wonder that most contradictory statements are given 

 as to the effect of red, green, and violet light upon related organisms, and 

 that great caution is demanded in drawing conclusions from the data at 

 hand. 



a. The Effective Rays in the Retardation of Growth by Light. 

 We have already seen (p. 166) that experiments upon the 

 effect of the different rays upon metabolism occupied the atten- 

 tion of naturalists in the first half of the century. It was but 



