428 EFFECT OF LIGHT [Ch. XVII 



a step to the study of the effect of these rays upon growth, and 

 this step was taken by Sachs in 1864. 



The method of subjecting the plant to particular rays was as follows : 

 The apparatus consisted of a glass jar, placed inside of a larger glass jar, the 

 interspace being filled with a colored fluid. This apparatus stood behind a 

 southeast window. Orange light was obtained by 12 to 13 mm. of a satu- 

 rated solution of potassium dichromate, which transmitted red, orange, and 

 yellow, but no blue or violet ; and blue light, by the same thickness of 

 ammoniated copper sulphate, which excluded all rays of shorter vibration 

 than the green, but, likewise, reduced the intensity of the violet end of the 

 spectrum. The relative chemical intensity of the light passing through the 

 solutions was determined by noting the time required to blacken photo- 

 graphic paper held in the inner jar. 



Under the conditions of the experiment young seedlings of 

 the white mustard, Brassica alba, and of flax, Linum usitatis- 

 simum, grew more rapidly and vigorously in the orange rays, 

 which act thus like darkness, than in the blue, which act thus 

 like daylight. In orange light the leaves, although differen- 

 tiated, remain small, while the internodes are elongated ; in 

 both of which respects the plants show themselves etiolated. 

 In the blue rays the cotyledons, unlike the leaves, remain small; 

 since, in the absence of assimilation, which requires red rays, 

 they are drawn upon for food. Throughout, the less intense 

 blue rays acted more like white light than the more intense 

 orange rays. 



Several confirmatory experiments upon other plants may 

 now be briefly considered. Bert ('78, p. 986) cultivated a 

 Sensitive Plant in a lantern made of red glass. It lived for 

 months, elongated considerably, and had small leaves ; one 

 might have called it etiolated but for its remaining green, 

 owing to the formation of chlorophyll in the red rays (p. 170, 

 note). Behind blue glass it had the general form character- 

 istic of white light, but it did not grow as large as in day- 

 light — which includes also the warmer rays. The whole 

 habit of the plants in the red rays indicated greater turges- 

 cence than in the blue : a result which Bert suggests may be 

 due to the manufacture in the presence of the red-yellow rays 

 of a material (glucose) causing an endosmotic flow. This 

 explanation, however, seems negatived by the fact that plants 

 grown in darkness exhibit this same condition. Weisner 



