EXTERNAL INFLUENCES AS CAUSES OF VARIATION 241 



from the blue upward and most pronounced in the invisible 

 "ultraviolet" portions of the spectrum. These particular wave 

 lengths seem closely akin to chemical energy, and their effect, 

 invisible and subtle as it is, should not be overlooked. 



Certain organic compounds are readily formed only by the 

 aid of light; thus the reaction C 14 H 8 O 2 +C 6 H 5 CHO=C 14 H 8 

 (O.H)(O.CO.C 6 H 5 ) takes place in sunlight, but in darkness the 

 substances are indifferent to each other. 1 It is under this same 

 principle that the vegetable substance chlorophyll is able to 

 break up the CO 2 of the atmosphere and fix the carbon in the 

 form of starch, setting free the oxygen. This is the most dis- 

 tinctive act of plant life, and yet it takes place only in the pres- 

 ence of light. The student is therefore prepared to realize that 

 light is one of the controlling forces, not only in effecting 

 chemical compounds in the non-living world but in the activi- 

 ties of living matter as well ; that in many respects its action is 

 fundamental (as in the fixing of carbon), in others incidental, 

 and in still others even accidental (as in the color of chloro- 

 phyll or of gold or silver). In any event it is an influence to 

 be taken account of when one is engaged in the study of the 

 circumstances that control the activities of living matter. 



Effect of light upon functional activity. 2 The effects of sun- 

 light upon growth are of three kinds, one due to the heat 

 rays of the lower spectrum, the others to the luminous and the 

 so-called chemical or "actinic" rays of the upper spectrum, from 

 the blue to a considerable distance beyond the violet. Strange 

 as it may seem, the influence of light upon the fixation of car- 

 bon is greatest in the thermic rather than in the actinic region 

 of the spectrum. Timiriazeff 3 kept a plant in the darkness 

 until the starch in the leaves had been absorbed. "Then in a 

 dark room a prismatic spectrum was thrown upon the leaf and 

 the position of Fraunhofer's lines indicated on the leaf. After 

 three to six hours starch had formed under the influence of the 

 light, only in the region of the absorption bands of chlorophyll 

 lying between B and D" as determined by treating first with 



1 C. B. Davenport, Experimental Morphology, Part I, p. 163. 



2 Ibid. Part I, pp. 166-180 ; Part II, pp. 416-436. 



3 Ibid. Part I, pp. 169-170. The italics are mine. 



