114 PHOTOSYNTHESIS 



that the chlorophyll apparatus is adapted to the conditions of illumina- 

 tion to which the plant is exposed. Thus a plant receives not only the 

 direct rays of the sun, but also dififuse light, which in composition has been 

 considerably altered by reflection and dispersion. While the direct rays 

 of sunlight are altered by absorption of the blue-violet rays in passing 

 through the atmosphere, in dififuse light the blue-violet rays are relatively 

 in excess. In the direct rays of the siin there is therefore a relative pre- 

 ponderance of red-yellow light while in diffuse light there is a preponder- 

 ance of blue light. This is based upon the well-known theory of Lord 

 Rayleigh. Stahl sees in the absorption of light by chlorophyll a direct 

 adaptation to this condition. The pigments of the leaf are of two dififerent 

 colored components capable of absorbing the different spectral components 

 of the light incident on them. Stahl considers that in the chlorophyll, the 

 blue-green pigment serves to absorb the red-yellow rays of direct sun- 

 light and the yellow pigment in leaves absorbs the diffuse reflected blue 

 rays. That the plant does not absorb the green and infra-red rays he 

 explains on the ground that in diffuse light these rays are of such low in- 

 tensity that the plant can make no use of them, while in direct sunlight 

 these rays are of such high intensity that their absorption would result in 

 an increase of temperature harmful to the leaf. Stahl considers that the 

 plant absorbs and utilizes those rays which are the most constant com- 

 ponents of the light to which it is exposed, at the same time avoiding 

 the absorption of rays which have little value for photochemical reactions. 

 These views are still largely speculative. It appears to be true that light 

 over a wide range of frequency is capable of inducing photosynthesis. 

 Interesting in this relation, it may parenthetically be stated, is the fact, 

 observed by Stern,*'' that the chlorophyll in the cell fluoresces. Chloro- 

 phyllous cells illuminated with yellow, green or blue light fluoresce red 

 light, 630-650 \i \\. and 660-690 |.i ^i, that is, light which is in the red absorp- 

 tion band of chlorophyll. 



In view of the fact that marine plants growing at some distance below 

 the surface are subjected to illumination of much more restricted spectral 

 regions than land plants, the theory of complementary adaptation has been 

 studied primarily in relation to marine life. It is also in these plants that 

 differences in color, due to pigments besides chlorophyll, are most striking. 

 According to the Engelmann theory the production of these pigments is 

 the reaction to a particular factor in the environment, the frequency of 

 the light in which the plants are growing ; and these pigments play an im- 

 portant role in the photosynthetic process of the plants. On the basis of 

 experimental work Richter ^° comes to a very different conclusion. Work- 

 ing with a variety of marine algae of different color and with light of vari- 

 ous intensities and frequencies he concludes that it is not the wave-length 

 of the light which determines the color of the algae but the intensity of 

 the light. While Gaidukow. already cited, stressed particularly the de- 



■^ Stem, Ber. bot. Ges., 38, 28 (1920). 

 "Richter, Ber. bot. Ges., 30, 280 (1912). 



