THE DETERMINING FACTORS IN PHOTOSYNTHESIS 33 



orange-red rays) are the most efficient in photosynthesis. But 

 in other plants such as the algae where other prominent pigments 

 are present besides chlorophyll, the greatest activity may lie in 

 a different region of the spectrum. In the blue-green algse, the 

 maximum activity is near D in the orange-yellow; in the brown 

 algse between D and E in the yellow-green and also between B 

 and C in the red; and in the red algae between D and E. Engel- 

 mann believed that the color of light most absorbed in photo- 

 synthesis was complementary to the color of the plant as shown 

 in the following table : 



Color of alga Light absorbed 



blue-green orange-yellow 



green reddish-orange 



brown (yellow) red and yellow-green 



red yellow-green. 



This is supported by the fact that the blue-green alga, Oscil- 

 latoria, undergoes chameleon-like changes of color (this, however, 

 is not the origin of the name !) depending upon the light in which 

 it is placed. Unlike the chameleon, however, it does not take on 

 the color of the light falling upon it but rather that of the comple- 

 mentary color. Thus in blue light it is brownish-yellow, in green 

 light it becomes red, in yellow light it turns to bluish-green, and in 

 red light it assumes a green color. This alga also contains a reddish 

 and a bluish-green pigment, and the complementary adaptation 

 seems to depend upon the arrangement and the amounts of the 

 various pigments in the cells. 



At any rate this ability to absorb other rays besides red is ex- 

 tremely important for algae growing in the ocean depths and is 

 directly connected with their distribution. Blue-green and green 

 algae are found nearest the surface in comparatively shallow water. 

 The browns are found deeper; while the reds occur at the great- 

 est depth of all. The red light so important to green algae penetrates 

 the water only a short distance, while the yellow and green light 

 so necessary for the brown and red algae may penetrate to a much 

 greater distance. It is interesting to note in this connection that 

 red algae when grown near the surface of the ocean are greener 

 than usual, not because they contain more chlorophyll (the re- 

 verse is actually the case), but because they contain less of the 

 red pigment. 



Intensity. — In all such matters where quantity enters into 

 consideration it is necessary to speak of the " cardinal points," — ■ 



