1030 



BIOLOGICAL EFFECTS OF RADIATION 



The absorption of some of the leaf pigments in the infra-red and 

 ultra-violet regions of the spectrum has been reported (14, 112, 127, 128), 

 but a discussion of these does not seem pertinent to this review, because 

 photosynthesis does not take place in these spectral regions. 



The absorption bands of a colloidal suspension of chlorophyll are not 

 so sharp and are shifted to the red as compared with the absorption 

 bands of a true solution of chlorophyll (34, 143, 147). The absorption 



bands of colloidal chlorophyll 

 very nearly coincide with those 

 of a living leaf. It has, there- 

 fore, been concluded that in 

 the leaf, chlorophyll exists in 

 a colloidal state. Against this 

 supposition, however, is the 

 fact that colloidal chlorophyll 

 does not fluoresce, whereas one 

 of the most striking properties 

 of chlorophyll in situ is its 

 characteristic fluorescence 

 spectrum (50) . It is, of course, 

 possible that in the leaf chloro- 

 phyll may be adsorbed on 

 certain colloidal cell constit- 

 uents, for it has been found 

 (84) that chlorophyll adsorbed 

 on kaolin, alumina, or globulin, 

 especially in the presence of 

 fats and lecithin, exhibits its 

 characteristic fluorescence. In the living cell the chlorophyll is 

 apparently combined in this manner with some protoplasmic protein 

 (66, 67, 79). 



In true solution chlorophyll a, chlorophyll 6, and carotene show 

 fluorescence (40, 58, 154). The fluorescence spectra of the chlorophylls 

 in ether solution are shown in Fig. 7 (154). 



The products of photosynthesis, as far as they are known, are optically 

 active. The recent discovery of StoU (119a) that both chlorophylls 

 exhibit optical activity is therefore of considerable interest. For freshly 

 prepared solutions [aj^o chlorophyll a (-flH20) = —262°, chlorophyll 

 b (-hlH20) = —267°. It is important that Stoll has demonstrated 

 that this optical activity of the chlorophylls arises from the portion of the 

 molecule which yields phaeophorbides and cannot be ascribed to the 

 phytol, the optical activity of which has recently been questioned (132). 

 The optical activity of chlorophyll is maintained during photosynthesis 

 though it disappears rapidly in solutions of chlorophyll in organic 



100 

 Wave Length in mji 



Fig. 7. — Fluorescence spectra in ether of A, 

 chlorophyll a; B, chlorophyll h. (Zscheile, 154.) 



