130 Carbon Assimilation. 



sucrose is a primary product in assimilation. Although it is possible 

 that all the sugar in the mesophyll cells of the leaf is sucrose, and 

 that the hexoses are confined to the vascular bundles, there is no 

 direct evidence of this, and there seems no sufficient reason to 

 conclude that sucrose is the first sugar formed rather than that 

 glucose or other hexoses first appear and that cane sugar is formed 

 from them. It is no more unreasonable to suppose that sucrose 

 should be formed from hexoses in the leaf when these latter reach 

 a certain concentration, thaf to suppose starch should be formed as 

 a temporary reserve carbohydrate in such leaves as those of 

 Troposolum or Potato, for as starch is a storage form in the potato 

 tuber, and is similarly formed in the leaf, so since sucrose is the 

 storage carbohydrate in the root ot Beta, being formed from hexoses 

 according to Davis, Daish and Sawyer, there is no reason why it 

 should not also be formed from hexoses in the leaf. The value of 

 evidence in regard to the first sugar formed in the leaf, derived from 

 considerations of the variation in amount ot the different sugars, is 

 indicated by the fact that Brown and Morris, Parkin, and Davis, 

 Daish and Sawyer all conclude that sucrose is the first sugar formed 

 in assimilation, while their results on which they base 

 this conclusion differ absolutely. Thus Brown and Morris 

 found that both the sucrose and hexose content diminished 

 during the day ; Parkin found the hexose content remained 

 practically constant, while the sucrose varied ; Davis, Daish and 

 Sawyer found both the sucrose content and hexose content 

 varying in the leaf, while in the veins the sucrose content remains 

 approximately constant, the hexose varying widely. 



We do not wish it to be supposed that we therefore support 

 the view that glucose is the first sugar of carbon assimilation. We 

 hold that the data so far produced from analyses of carbohydrates 

 in leaves and from microchemical examination provide insufficient 

 evidence in favour of or against either theory. While we may 

 regard starch as a secondary product of assimilation, and while also 

 there is good evidence that carbohydrates are translocated, as hexose 

 sugars, in some cases, or to some extent at any rate, and while 

 there is strong evidence that sugars are the first definitely known_ 

 products of the assimilatory process, there is not evidence at present 

 as to which particular sugar is the first one to be produced in the 

 leaf. 



