THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. I 



the following results of his experiments in detail, 

 per cent, of the dried leaf substance. 



The numbers indicate grams 



Expt. l,a. Leaves analysed at 5 a.m. 



I,b. Leaves cut off at 5 a.m. ; examined after 12 hours of assimilation. 



I, c. Leaves left on the plant and examined after 12 hours of assimilation. 

 Expt. II. Leaves left on the plant : (a) analysed at 9 a.m. ; (b) at 4 p.m. 

 Expts. Ill and IV. Leaves cut off: (a) analysed in the forenoon ; (b) 24 hours later— after 

 they had been kept meanwhile in darkness. 



These experiments are of the greatest interest, because they form the most 

 detailed quantitative estimate as yet made and because they form a basis for 

 further inquiry; their significance is not, however, so obvious. The carbo- 

 hydrates, which occur in the leaf, group themselves obviously into two series 

 possessing quite distinct characters : — 



1. Those which increase in a marked degree during assimilation in light 

 (I and 11), viz. cane sugar, maltose, starch (di- and polysaccharides). 



2. Those which increase in darkness (III and IV), viz. dextrose, levulose 

 (monosaccharides). 



This result, at first sight, appears somewhat surprising, since one would 

 expect that the simpler sugars (dextrose and levulose) would appear first in 

 the course of assimilation. Doubtless that is the case, but they become at 

 once further altered and in consequence do not accumulate at all. The con- 

 version takes place in at least three ways ; in the first place these hexoses 

 migrate out in very large quantities (compare I, c. with I, b.), next they become 

 respired, and, lastly, from them arise cane sugar and starch. In addition to 

 the construction of complicated carbohydrates a counter-formation of hexoses 

 also takes place, and this preponderates in the dark (III and IV). If we assume 

 that the dextrose and levulose are the first products of assimilation, then we 

 must consider cane sugar and starch as reserve substances, which are temporarily 

 stored by the assimilating cells, because the translocation of the assimilatory 

 products does not take place so rapidly as their manufacture. Many of the 

 features indicated in the tables are as yet inexplicable, and into the details of these 

 we need not go ; possibly, further research may afford an explanation of them. 

 As to the relations subsisting between the di- and polysaccharides. Brown 

 and Morris have advanced the view that the starch arises from the cane sugar 

 and the maltose from the starch. This view can scarcely be held as correct, 

 and, moreover, it leaves unexplained many observed facts. 



The assumption made by these authors that the whole of the carbon passes 

 over, in the first instance, at once into cane sugar is, at least, not proved ; doubt- 

 less this sugar is rapidly removed elsewhere. There is also the possibility 

 that the carbon, during assimilation, is at once united with nitrogen and that 

 the carbohydrate is formed by subsequent dissociation of this compound. 

 A. Meyer (1885) held that the construction of proteids during the process of 

 assimilation was not unlikely, but he was unable to support his view by any 

 conclusive evidence. Saposchnikoff (1895) has dealt with this question in 

 greater detail. He determined the proportion of the dry substance formed during 



