THE NATURE OF PHOTOSYNTHESIS 113 



of light as a cultural condition has already been alluded to ; its effect on 

 plants under natural conditions has been a subject of long investigation 

 from which some important conclusions have already been drawn. 



Englemann ^^ first offered the hypothesis that the color of certain 

 marine plants was complementary to the color of the light to which they 

 were exposed. It is a familiar fact that water has a higher absorption for 

 the longer wave-lengths of light than for the shorter wave-lengths. As a 

 consequence the deeper the water, the greater is the relative proportion of 

 blue light. Thus, if 1000 represents the light intensity at a depth of one 

 meter the distribution of the different wave-lengths is about as follows : ^* 



Blue 

 Green Blue Violet 



230 450 866 



0.0003 0.001 0.003 



— — 0.0001 



Shelford and Gail ®' found that the brown algal zone is at a depth of 

 5 to 20 meters where the shorter wave-lengths have an intensity of about 

 10 per cent of full sunlight and the longer, red wave-lengths about 1 per 

 cent. The red algal zone lies between 10 and 30 meters in depth where 

 the blue light is 2 to 10 per cent of full sunlight and the red light is 0.032 

 to 1 per cent. 



Gaidukow *" reported that he was able to alter the color of certain 

 marine algae by growing them under colored light and thus obtained plants 

 with a color complementary to the color of the light in which they were 

 grown. These and other similar observations gave rise to the theory of 

 complementary chromatic adaptation. The theory and some of the rami- 

 fications and deductions which have been drawn therefrom have been at- 

 tacked from different sides, a discussion of which would lead too far 

 afield.^^ Some of the principles involved are, however, of direct bearing 

 on the photosynthesis problem. In its broader significance, of course, 

 the question resolves itself into the old one of why vegetation is green. 



This question has been subjected to an interesting theoretical analysis 

 by Stahl. The theory is really an extension to land plants of Engle- 

 mann's theory of the distribution of marine plants. Stahl ^^ points out 



"=• Englemann, Bot. Zeitg., 42, 81 (1884). 



'* Monaco, The Prince of, Sci. Monthly, 13, 177 (1921). 



^ Shelford, V. E., and Gail, F. VV., Pub. Puget Sound Biological Station, 1920, 

 141. Gail, F. W., ibid., 177-193. Geitler, I., Int. Rev. d. gcs. Hydrobiol Hydro., 10, 

 681 (1922). Pascher, Bot. Arch.. 3, 311 (1923). 



^Gaidukow, Bcr. bot. Ges., 21, 484, 517 (1903) ; 24, 1, 23 (1906). 



"Baresch, K., Jahrb. zmss. Bot., 52, 145 (1913) ; Bcr. hot. Gcs.. 37, 25 (1919) ; 

 ibid., 39, 93 (1921); Zeit. f. Bot., 13, 65 (1921). Magnus, W., and Schindler, B., 

 Ber. hot. Gc.^., 30, 314 (1912). Pringsheim, E., Cohn's Beitr. z. Biol. d. Pnanzen. 

 12, 49 (1914). Tobler, F., Die Naturimssenschaftcn. 1, 845 (1913). Savageau, C., 

 Compt. rend. Soc. bioL, 64, 95 (1908). 



^^ Stahl, "Laubfarbe und Ilimmelslicht," Jena, 1906. 



