The Mechanism of Carhon Assimilation in Green Plants. 319 



of the products be rapidly removed from the sphere of action. The physical 

 conditions were first enunciated by Timiriazeff, in the Croonian Lecture 

 for 1903, and are as follows : — (1) The optical sensitiser must be in the 

 form of an extremely thin film ; and (2) the concentration of the sensitiser 

 in this film must be so great as to ensure a large transformation of the 

 incident solar energy within a very small space. The fulfilment of both 

 these conditions in the case of chlorophyll in the plant is a matter of 

 common knowledge, and the experiments to be described were designed 

 to reproduce as far as possible the essential mechanical, physical, and chemical 

 arrangements which obtain in the green leaf. 



It should be mentioned here, as the result of some observations by one 

 of us in conjunction with Miss Irving* on the structure of the large 

 chloroplasts of Selaginella and Chlorophytum, that the chlorophyll is 

 restricted to the exterior of the granule, and that the thickness of the film 

 is about 2*5 x 10 -3 mm. Whether this applies generally or not is of no 

 consequence in this connection, the essential point being that we have here 

 an actual workino; arrangement, and one which can be imitated without 



DO',' 



difficulty. 



The greatest possible concentration of chlorophyll is obtained when the 

 solvent used for extracting it has been completely driven off, and if material 

 is employed which contains very little else that is soluble in the liquid 

 in which the final solution of the chlorophyll takes place ; one may regard 

 the concentration of the chlorophyll thus obtained as being approximately 

 the same as in the plant. For these experiments the material used was the 

 leaves of grass or wheat, chosen on account of their comparative freedom 

 from fats and oils. Alcohol was used for the extraction, and petroleum 

 ether for the final solution of the chlorophyll. 



In the first series of experiments pieces of glass plate, 5x4 inches, were 

 covered with an aqueous solution of gelatine so as to form a layer 1 to 2 mm. 

 thick. When set, this was painted over with a solution of chlorophyll in 

 petroleum ether or benzene. In this way a fairly uniform film of 

 chlorophyll is obtained, and if, when the plate is placed in carbon dioxide, 

 light is allowed to strike it in the direction —> carbon dioxide chlorophyll 

 — * gelatine, we have a reproduction of the essential features of the arrange- 

 ment in the living cell. Measurements of the thickness of the chlorophyll 

 film gave an average value of about 6 x 10 -3 mm. 



On placing plates prepared in this manner in a bell jar containing moist 

 carbon dioxide and exposing to light, the chlorophyll in the course of a 

 few hours, became completely bleached, and the gelatine developed a red 



* At present unpublished. 



2 c 2 



