124 



Carbon Assimilation. 



(iv.) In the first stajje of growth, practically all the reducing 

 sugars and about half the sucrose disappear from the leaf during 

 the night. As the season proceeds, a less proportion of the sugar 

 in the leaf disappears each night. It is especially the hexoses 

 which increase in the leaf owing to this. These results are 

 summarised in the following table. 



Table XXXV. 

 Sensoiial Vay'iations in Carbohydrate Content of Mangold Leaves. 



The observations made by Davis, Daish and Sawyer on the 

 sugars of midribs and petioles, show that these always contain a 

 higher percentage of sugars than the leaves, and this percentage 

 increases with the season. The hexoses are always much in excess 

 of the sucrose, and the ratio of hexoses to sucrose is always much 

 {greater in the petioles than in the leaf lamina. These results are 

 comparable with Parkin's observations that the sugarcontent of the 

 snowdrop leaf increases from above downwards, and that the ratio 

 of hexose to sucrose also increases. The conclusion drawn from 

 this by both Parkin and the Rothamsted workers is that sucrose is 

 the first sugar formed in the leaf and that this is converted into 

 hexoses for translocation purposes. In support of this they also 

 adduce the fact that the cane sugar is always present in relatively 

 high proportion in the leaf, especially early in the season when it is 

 present in excess of the hexoses. They suppose the cane sugar is 

 gradually inverted by means of the enzyme invertase which is 

 secreted or distributed on the surface of the sieve tubes. 



We have already referred to the unreliable character of the 

 determinations of glucose and fructose, an unreliability which is 

 quite realised by Davis, Daish and Sawyer. As, therefore, it is not 

 at all clear what the quantities they term " apparent dextrose " and 

 " apparent levulose " really represent, we do not think any useful 

 purpose would be served by discussing the values they obtain for 

 these quantities. 



