76 Scientific Proceedings^ Royal Dublin Society. 



The action apparently occurs within the molecule of the chlorophyll itself. 

 Possibly electrons are transferred from one atom to another, thus altering 

 the linkage, and hence the chemical nature of the molecule, or its atomic 

 groups. 



It is also conceivable that electrons might be transferred from molecules 

 of chlorophyll to adjacent molecules, whose presence is necessary to facilitate 

 the escape. Such an action could hardly be classed as photo-electric, using 

 the term in its ordinary sense, and would not be detected by the method 

 employed. 



If it is shown that chlorophyll acts like other sensitizers, and affects the 

 sensitiveness of the photographic plate even when it is removed before 

 development [Abney (1)], such a transference of electrons from the chloro- 

 phyll to one of the other constituents of the photographic film seems a 

 necessary assumption. 



The constant association of the four pigments 



Chlorophyll a (CssHvaOjNjMg), Chlorophyll I (CaiHroOeNiMg), 

 Xanthophyll (C40H56O2), and Carotin (C4(|H56) 

 in sensibly constant proportions, in the ehloroplasts of green plants, as 

 shown by Willstatter and Stoll(9), and the chemical relationship of these 

 substances, elucidated by the same investigators, pointedly suggest a trans- 

 ference of electrons between the components of the pigment complex in the 

 chloroplast securing the ultimate reduction of the carbon dioxide and the 

 liberation of oxygen. This transference must be effected directly or indi- 

 rectly by the light. Any evidence as to the nature of this process and the 

 interdependence of these substances is so desirable that it is hoped shortly 

 to repeat our observations on pure samples of the individual pigments, 

 instead of using the crude leaf-extract or leaf-powder. It seems, however, 

 rather unlikely that a positive result will be obtained with the separate pig- 

 ments, since the result with the complex was so decisively negative. 



For the present, then, it appears we must assume that the atomic groups 

 of the leaf-pigment enter into the reaction of photosynthesis, and participate 

 in the combinations and decompositions which ultimately lead to the forma- 

 tion of carbohydrates and the evolution of oxygen. The experimental 

 evidence seems to debar us from regarding the pigments in the ehloroplasts 

 as mechanical contrivances for effecting the ionization of molecules external 

 to themselves. Thus the chemical theories of photosynthesis, such as 

 Hoppe Seyler's and Willstatter's, which assume that the chlorophyll itself 

 enters into the reactions, are to be preferred to those suggestions, like 

 Siegfried's, which suppose that the reaction is accomplished externally to the 

 chlorophyll by means of the energy absorbed and transformed by the latter. 



