1028 



BIOLOGICAL EFFECTS OF RADIATION 



500 428 375 

 "*" Wcjve Lengths in mfi. 



Fig. 3. — Absorption curves in benzene of A, 

 chlorophyll a; B, chlorophyll b. {Winter stein and 

 Stein, 145.) 



Of primary importance is the question as to what pigments are of sig- 

 nificance for the photosynthetic reaction. This question can be partially 

 answered by a comparison of the absorption spectra of the photosyn- 

 thetic organs and of the pigments themselves with the photo.synthetic 



activity of different wave- 

 lengths of light of the same 

 incident intensity. 



In order to avoid the 

 influence of the physical 

 structure of the leaf on the 

 determination of the absorp- 

 tion spectra of leaf pigments 

 many investigators have used 

 extracts of the leaf pigments 

 in organic solvents. In 

 general these solutions show 

 the same type of absorption curve as the leaves themselves, which 

 are characterized by strong absorption in the red and violet and high 

 transmission in the green. In acetone and alcohol extracts of leaves the 

 absorption curves are shifted toward the violet as compared with the 

 absorption curves of the living 

 leaves, Fig. 1. The narrow 

 absorption bands character- 

 istic of pure chlorophyll solu- 

 tions are not evident in the 

 absorption curves of leaves. 



On the basis of chromato- 

 graphic adsorption ex- 

 periments the principal 

 chloroplast pigments are: 

 chlorophyll a, chlorophyll h, 

 a- and /3-carotene, and the 

 xanthophylls (144). The pig- 

 ment content of leaves varies 

 with the conditions under 

 which the leaves are obtained. 

 On the basis of percentage 

 dry weight the amount of 

 chlorophyll is greater for 

 shade than for sun plants, 

 whereas the amount of yellow 



700 



800 



500 600 



Wave Length in mfi 



Fio. 4. — Absorption curves in ether of A, chloro- 

 phyll o; B, chlorophyll b. (Zscheile, 153.) 



pigment is approximately the same (142, page 112). The total chloro- 

 phyll varies from about 0.5 to 1.2 per cent and the total yellow pigments 

 from about 0.1 to 0.2 per cent, in a number of leaves. The ratio of 



