THE MECHANISM OF ORGANIC SYNTHESIS 123 



03 per cent, carbon dioxide at 100, the assimilation in an atmosphere 

 containing 1 per cent, was 237, and was not increased by raising the 

 percentage of carbon dioxide to 7 per cent. Owing to the decomposi- 

 tion of the organic matter of the soil, the percentage of carbon dioxide 

 near the ground is always greater than in the higher strata of the 

 atmosphere a fact which is taken advantage of by the low- growing 

 plants and herbage. Other necessary conditions 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 latter 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 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 trans- 

 mitted 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 chloro- 

 phyll 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 pronounced 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 

 physiologists are still undecided. There can be no doubt that an early 

 product of the process is a hexose, which is rapidly converted into car e 

 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 deriva- 

 tives of formaldehyde as sodium oxymethyl-sulphonate, or from 

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



