THE MECHANISM OF ORGANIC SYNTHESIS 109 



evolution of oxygen will go on in the isolated chloroplast. In the absence 

 of chlorophyll, as in an etiolated leaf, the formation of starch will take place 

 if the plant be supplied with a sugar such as glucose, and this conversion 

 represents the main function of the leucoplasts present in all the cells of the 

 reserve organs of plants. In the absence of chlorophyll no decomposition of 

 carbon dioxide takes place, so that this pigment is evidently essential for 

 the utilisation of the sun's energy. Chlorophyll may be extracted from 

 leaves by means of absolute alcohol. A solution is thus obtained which is 

 green by transmitted and red by reflected light, i.e. chlorophyll is a fluorescent 

 substance. It presents four absorption bands, the chief being an intense 

 black band between Fraunhofer's lines B and C. If the chlorophyll is the 

 means of conversion of the solar into chemical energy, the conversion must 

 take place at the expense of the light which is absorbed by the pigment. 

 One would expect therefore the process of assimilation to be most' pro- 

 nounced in those parts of the spectrum corresponding to the absorption 

 bands an expectation which has been realised by experiment. 



As to the exact chemical changes effected by these absorbed rays physio- 

 logists are still undecided. There can be no doubt that an early product of 

 the process is a hexose, which is rapidly converted into cane sugar or into 

 starch. It was suggested by Baeyer in 1870 that carbon dioxide was 

 reduced to formaldehyde, which later by condensation yielded sugar. We 

 know that formaldehyde easily polymerises to form a mixture of hexoses, 

 but until recently no evidence had been brought forward of its presence as 

 an intermediate product in the assimilatory process. For most plants, 

 indeed, formaldehyde is extremely poisonous, though certain algse, as well 

 as the water-plant, Elodea. can stand a solution containing -001 per cent, 

 formaldehyde. Bokorny stated that spirogyra could form starch out of such 

 derivatives of formaldehyde as sodium oxymethyl-sulphonate, or from 

 methylal. The difficulty in these cases is that possibly a spontaneous 

 formation of sugar from the formaldehyde had taken place in the solution and 

 that the plants were using up the sugar rather than the formaldehyde as the 

 source of their starch. 



One must assume, with Timiriazeff, that the function of chlorophyll in 

 the process of assimilation is that of a sensitiser. Just as the addition of 

 eosin to the emulsion used for coating photographic plates will render these 

 sensitive to the red and green parts of the spectrum, i.e. will excite change in 

 the silver salt when light from these parts of the spectrum falls upon it, so 

 the chlorophyll serves as a means by which the absorbed solar energy can be 

 utilised for the production of chemical change in the chloroplast. Attempts 

 have been made to imitate this process outside the plant. Thus Bach passed 

 a stream of carbon dioxide through a 1-5 per cent, solution of a fluorescent 

 substance, uranium acetate, in sunlight. As a result there was a precipitate 

 of uranium oxide and peroxide, with the formation of traces of formaldehyde. 

 Usher and Priestley, on treating a solution of carbon dioxide with 1-5 per 

 cent, uranium "acetate or sulphate in bright sunlight, obtained uranium 

 peroxide and formic acid, but no formaldehyde. The formation of peroxides 



