t)ixoN AND Ball — Photosynthesis and the Electronic theory. 439 



liquid air. On development satisfactory sensitization was apparent. The 

 development with rodinal lasted in each case about three minutes. 



These results, which were repeated several times, establish the fact that 

 sensitization with chlorophyll of photographic films, whether of collodion or 

 of gelatine, is effective at the temperature of liquid air. Inasmuch as in these 

 experiments it is only light of visible wave-lengths, viz., A = 6'\5f.ifi - 725ju//j 

 which is absorbed by the chlorophyll, and produces the ionization forming the 

 latent image, it follows that this light displaces electrons of the chlorophyll. 

 Previous experimental work has shown that visible light does not expel 

 electrons from chlorophyll.^ Hence it follows that the energy absorbed 

 from visible light is wholly or in part used in the displacement of electrons 

 within the chloroi^hyll molecule, making some atomic group or groups within 

 it reactive. In photosynthesis, we may suppose that these groups react with 

 the raw supplies, viz., carbon dioxide and water. 



We might imagine the process to take place as follows : — 



C,5Hj,0,N,Mg + CO, = C,6H,oOeK,Mg + CH.O 

 (chlorophyll ft) (chlorophyll b) 



C,5H,oOeN,Mg + H,0 = C5,H,,06N,Mg + 0, 

 (chlorophyll b) (chlorophyll a). 



Such a scheme would account for the constant proportions of chlorophyll 

 a and b. The fact that the formation of formaldehyde cannot be detected 

 when chlorophyll in presence of carbon dioxide alone is exposed in vitro (3) 

 may be explained by assuming, with Siegfried (6) and Willstatter (8), that 

 the fixation of carbon dioxide with a protein is first necessary, and that the 

 decomposition of the carbamino-acid or carbaminate is effected by the reactive 

 group in the chlorophyll produced by the absorption of light. The formation 

 of chlorophyll h and formaldehyde from chlorophyll a and this loosely com- 

 bined carbon dioxide would probably take place instantaneously on exposure 

 to light. A momentary inflow of carbon dioxide to make up the loss of 

 carbamino-acid or carbaminate might be expected ; but the subsequent rate 

 of absorption would depend on the rapidity with which the carbamino-acid 

 was broken down by the available chlorophyll a, and would gradually rise to 

 a steady state as the normal proportion of chlorophyll a to chlorophyll b was 



' It is of interest here to record an unpublished observation of J. Joly and J. H. J. 

 Poole. These investigators, using an extremely sensitive method of investigation — the 

 tilted leaf electroscope — found that another sensitizer, viz. , Sensitol Red, like chlorophyll, 

 is practically non-photoelectric under the action of visible light. 



