CHROMATIC ADAPTATION 35 



periments, investigated the quantity of radiant energy ab- 

 sorbed in relation to the work done in carbon assimilation. 

 The ratio between these two values is a measure of the efficiency 

 of the organism in this particular connection ; thus if the rad- 

 iant energy absorbed by the plant be E and the energy utilized 

 be U, then the ratio U/E will give an index of efficiency. The 

 greater the value of E, the smaller is the ratio ; with decreasing 

 intensity of E the value rises until it reaches a limit, which 

 limit marks the point of maximum efficiency. Using a culture 

 of Chlorella, which had been grown in uniform conditions 

 with a low light intensity, and keeping the illumination limit- 

 ing, a mean value of 59 per cent, and a maximum value of 

 63-5 per cent, was obtained in a red light of a spectral range 

 570-645 pn. Warburg and Negelein also compared the 

 photosynthetic activity of the same plant illuminated with 

 light of different wave-lengths. The accompanying table 

 summarizes their observations : — 



Light. Per Cent. Efficiency. 

 Red . 610-690 fifi 59 



Yellow . 578 fin 53'3 



Green . 546 (j-h 44-4 



Blue . 436 nn 33-8 



It will be seen that the efficiency decreases with the decrease 

 in the wave-length. Also, these values differ from those 

 obtained by other investigators, mentioned above, using dif- 

 ferent plants and different methods, a result to be expected 

 for the reasons earlier given. 



Chromatic Adaptation. — This consideration of the quality 

 of light naturally introduces the problem of chromatic adap- 

 tation. As is well known, the colour of deep water is green 

 owing to the greater absorption by the water of the long rays, 

 which are strikingly absorbed and utilized by the chloroplasts. 

 A plant growing in deep water, therefore, is handicapped by 

 the different quality of light. Many marine alga; have, in 

 addition to their chlorophyll, water-soluble pigments — phyco- 

 erythrin for example * — which are complementary in colour 

 to that of the surroundings and thus are able to increase the 



* See Vol. I, p. 353- 



3* 



