166 LIGHT AND PROTOPLASM [Cn. VII 



3. THE EFFECT OF LIGHT UPON THE GENERAL FUNCTIONS 



OF ORGANISMS 



In this section we shall consider in succession (1) the effect 

 of light upon metabolism ; (2) the vital limits of light action 

 on protoplasm ; and (3) the effect of light upon the movement 

 of protoplasm. 



1. Effect of Light upon Metabolism (including Assimilation). 

 Metabolism is a complex of chemical processes. Since, as 

 we have already seen, light has important chemical effects, we 

 are not surprised to find that it plays an important role in 

 metabolism. The effects of light are, however, of two distinct 

 kinds. One is a thermic effect, due to the heat rays of white 

 light ; ' the other is a chemical effect due to the " actinic rays " 

 of the spectrum. 



a. The Thermic Effect of Light on Metabolism is shown chiefly 

 in the assimilative processes of chlorophyllaceous plants. The 

 facts of this assimilation are chiefly these : various simple com- 

 pounds, water, carbon dioxide, salts of ammonia and nitrates, 

 are used as food by plants. For every volume of the gas - 

 carbon dioxide taken in, one volume (nearly) of oxygen is 

 excreted. Starch (C 6 H 10 O 6 ) is the first visible product of the 

 water and carbon dioxide taken in. Chlorophyll is essential to 

 the absorption of carbon dioxide, to the giving forth of oxygen, 

 and to the formation of starch. Finally, chlorophyll can as- 

 similate only in the presence of sunlight and at a proper tem- 

 perature. 



Now, not all the rays of sunlight with their varied wave 

 lengths are essential to this process. Just what rays are 

 the essential ones has been a point of some dispute. The 

 earlier studies on the subject, made chiefly by DRAPER ('44), 

 SACHS ('64), and PFEFFER ('71), were unanimous in declaring 

 that the most active rays in assimilation were those occupying 

 the yellow part of the spectrum at about line D the region 

 of maximum brightness to our eyes (Fig. 42). But these ob- 

 servers were at fault in that, while they carefully determined 

 the quality of light and the corresponding quantity of assimila- 

 tion, none of them gave, in the experiments with color screens, 

 any adequate data upon the intensity of the diversely colored 



