442 Prof. C. Timiriazeff. [Apr. 30,. 



solutions of protophyllin possess a colouring'power greatly inferior to 

 that of chlorophyll, so that their presence may be' easily disguised by 

 the latter. Thus assuming that this reduction of chlorophyll actually 

 takes place in a green leaf, it will hardly be manifested by a change of 

 colour or of the spectrum, but simply by a diminution in the intensity 

 of the green hue, by a certain bleaching of the green' parts exposed to 

 the light. Now this bleaching effect has in fact been often observed, 

 but was generally attributed to the migration or change of volume of 

 the chloroplasts a supposition that seems to me highly improbable 

 when applied to the palisade-parenchyma, especially when the rays of 

 light do not fall strictly in the direction of the cell axes* I really think 

 that at any rate the greater part of this bleaching may be accounted for 

 by supposing some of the chlorophyllin to be reduced to protophyllin. 

 To sum up, we may thus admit the existence not only of two, as in the 

 blood, but even of three states of oxidation of the green colouring 

 matter. These transformations may be considered a starting point for 

 a theory of chlorophyll as a chemical sensitiser. 



Having enumerated the chief arguments in favour of the admission 

 that chlorophyll may be considered a sensitiser in both acceptances of 

 the word a chemical and an optical sensitiser, an absorbent of the 

 products of reaction as well as an absorbent of the active rays I wilt 

 try to take a step further and show that it may be considered not only 

 one of the innumerable representatives of the group) but a substance 

 quite exceptionally adapted to this function. 



At the very outset of my researches I was impressed with the idea 

 that such an intensely endothermic reaction as the dissociation of 

 carbon dioxide, must needs stand in some relation to the energy 

 of the radiations involved in the process. At the time we had 

 no direct knowledge of the distribution of energy in the spectrum, but 

 I pointed out that researches in the diffraction spectrum might possibly 

 alter the current notions concerning the relative calorific effect of the 

 different radiations and prove that the greatest energy should be 

 attributed, not to the dark infra-red rays, but to a certain group of rays 

 in the visible spectrum possibly the same that are absorbed by the 

 green plant, so that the two maxima of energy and of chlorophyll 

 absorption may after all coincide. 



My previsions were fully confirmed by the brilliant researches of 

 Mr. Langley and Sir William Abney. I called attention to this fore- 

 seen coincidence in a small note in the ' Comptes E-endus ' shortly after the 

 appearance of Mr. Langley's classical paper. My statements of these 

 facts being still contested, the correctness of my quotations being even 

 suspected by Professor Engelmann, I feel myself bound to bring forward 

 the original data. In Sir William Abney's paper in the * Proceedings 

 of the Royal Society 'for 1887, it is shown that the maximum of energy 

 corresponds to the wave-length about two-thirds between 600 and 



