THE MECHANISM OF ORGANIC SYNTHEs ffi , , , 



advantage of by the low-growing plants and herbage. Other necessary c 

 ditions of assimilation are the presence of water and the maintenance of a 

 certain external temperature. The absorption of the sun's rays by the leaf 

 raises the temperature of the later above that of the surrounding medium 

 and so quickens the process of assimilation. 



The assimilation of carbon dioxide, the formation of starch, and the 

 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 kucoplasts 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 algae, 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 m 

 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 thes< 

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

 the silver salt when light from these parts of the spectrum falls upon il 

 the chlorophyll serves as a means by which the absorbed solar energy c 

 utilised for the production of chemical change in the chloroplast 



