432 EFFECT OF LIGHT [Ch. XVII 



h. The Effective Rays in the Acceleration of Crrowth ly 

 Light. — We have seen that the eifective rays in retardation 

 are the blue ; the question now arises : Will the same rays 

 serve, in other cases, for acceleration, or must the latter be 

 due to the red rays ? 



As concerns the germination of ferns we have the observa- 

 tions of Borodin ('68, p. 435), in the case of Aspidium, that 

 germination did not take place in the blue any more than it 

 did in darkness, while the red rays produced nearly the whole 

 effect of white light. Germination in this case seems to 

 demand the red rays for its processes — processes consisting 

 largely of certain chemical changes favoring imbibition of 

 water. Probably those seeds which germinate more rapidly in 

 the light than in the dark make similar use of the red rays, as 

 JoNSSON ('93) has, indeed, found they do in the case of Poa. 



Passing now to animals, we find the first critical work on 

 these organisms is that of Beclabd ('58). This experimenter 

 placed at one and the same time, under diversely colored glass 

 bells, eggs of the flesh-fly, ]Musca carnivora, taken from a 

 single laying. All the eggs produced larvEe ; of these, the 

 largest were formed under the violet or blue glass, the small- 

 est under the green. The effect of the other colors was inter- 

 mediate and fell in the following order : violet, blue, red, 

 yellow, white, green. The larvse reared under the violet rays 

 were three-fourths greater than those reared under the green.* 

 The apparent acceleration in violet, as compared with white 

 light, indicates that the green rays of white light retard. This 

 was the result actually obtained by Schnetzler ('74, p. 251), 

 who found that tadpoles developed more slowly behind green 

 glass (from which red and violet rays chiefly were cut out) 

 than behind clear glass. 



Yung ('T8) made more critical experiments upon tadpoles. 

 Screens of nearly monochromatic solutions were used (p. 157). 

 The size of the vessels and the number of tadpoles in each 



* Exactly opposite results for blow-flies are given by Davidson ('85), whose 

 work, however, strikes one as crude. Fly larvae reared in a bottle made of blue 

 glass had at the end of nine days only half the weight, on the average, of larvae 

 reared either behind clear glass or in the dark. This subject needs careful 

 investigation. 



