RADIANT ENERGY 33 



twice as much carbon dioxide as can Primus, but in so doing it 

 requires twice the amount of illumination. The difference in the 

 two leaves lies in their having different coefficients of accelera- 

 tion of their assimilatory activity with increased temperature. 



Of the use made by the plant of specific parts of the spec- 

 trum, it has been generally accepted that those wave-lengths 

 associated with the prominent bands in the red of the absorp- 

 tion spectrum of chlorophyll were the only ones concerned 

 with carbon assimilation. It is. however, to be remembered 

 that the light absorbed by such a structure as a leaf is not 

 necessarily all employed in the photosynthetic process, that rays 

 of different wave-lengths are not equally absorbed and that 

 different wave-lengths have different energy values. Further, 

 the most superficial chloroplasts are illuminated by practically 

 unaltered daylight, but those more deeply situated receive 

 light from which certain rays have been abstracted in various 

 degrees by the more superficial plastids and thus is of different 

 quality as compared with normal daylight. Wherefore, 

 in relevant investigations, light intensities must be known ; 

 the quality of light, i.e. its range in wave-lengths, be ascer- 

 tained ; and light must be, as far as is possible, the factor 

 determining the rate of the process. Since radiant energy 

 must be converted into chemical energy, it is obvious that the 

 essential requirement is the amount of energy used, or at 

 any rate available for the use of the plant ; in other words, 

 it is profitless to consider the intensity of carbon assimilation 

 in different coloured lights without determining their energy 

 values. 



Ursprung * found that radiant energy of any wave-length, 

 including the greater part of the ultra-violet, is capable of 

 inducing starch formation in green leaves, provided that a 

 sufficient time be allowed. He even found starch to be formed 

 on exposure to infra-red rays ; it is, however, not clear whether 

 this starch was a direct product of carbon assimilation since 

 starch can be formed in the dark from pre-existing sugars. 

 Ursprung found in Phalaris arundinacea var. picta, which 



* Ursprung : " Ber. deut. bot. Gesells.," 1917, 35. 44 '• x 9i8, 36, 73> 

 86, in, 122. 



VOL. II. — 3 



