W. A. Davis, A. J. Daish and (>. C. Sawyer 303 



Discussion of Results. 



What is the primary sugar formed in photosynthesis in the leaf, 

 and in what form are the sugars conveyed to the root ? 



The results obtained above for the increase of the ratio of hexose to 

 saccharose in passing successively from the leaf to mid-ribs and stalks 

 suggest unmistakably that the cane sugar is formed in the leaf and 

 undergoes a regular and increasing amount of inversion as it passes 

 downwards to the root. Thus at the September picking (4 p.m.), 

 when there was 11 times as much hexose sugar as saccharose in the leaf, 

 the mid-ribs contained 3| and the stalks 5 times as much reducing 

 sugar as cane sugar. Thus as an average of the whole length of the 

 stalks the cane sugar formed only \ of the total sugars ; bearing in mind 

 the results in Table V, which show how rapidly the proportion of hexose 

 increases in passing from the top to the bottom halves of the stalks, 

 it is clear that the sap in immediate proximity to the root contains 

 probably only Jjj or ^l even of its total sugars in the form of cane sugar. 

 This fact taken in conjunction wdth the relatively high proportion of 

 the cane sugar in the leaf ^ suggests the almost irresistible conclusion that 

 the cane sugar is formed as a primary product in the leaf and is con- 

 verted into invert sugar for the purpose of rapid transit ; this conversion 

 takes place apparently in the veins of the leaf, in the mid-ribs and in the 

 stalks, as the sap travels on its way to the root, and the proportion of 

 sugar inverted steadily increases as the sap nears the root, until in its 

 immediate neighbourhood practically the whole of the sugar is in the 

 form of reducing sugar. The gradual inversion of the cane sugar is 

 probably brought about by the enzyme invertase which is secreted by 

 or distributed on the surface of the sieve tubes, which seem to be the 

 main channels by which the sugar is conveyed to the root (see Peklo 

 [1908]). In accordance with this view is the fact observed by Robertson, 

 Irvine and Dobson [1909]^ that invertase is abundant in the leaf and 

 stem of Beta vulgaris, but is absent from the root. 



' It must be borne in mind that the leaf tissue analysed contained aU the smaller 

 veins which could not be removed by the rough method we used ; only the large primary 

 mid-ribs were removed. It is probable therefore that the hexoses foimd in our leaf 

 analyses were mainly present in these veins. 



- CoUn [1915] has independently shown the presence of invertase in the leaves and 

 petioles of the leaf of Beta iiilijnris; he has established moreover the important fact, that 

 the proportion of invertase is greatest in the leaves, less in the upper part of the stalks 

 and practically nil at the base of the stalk, where it enters the root. In tlie root itself 

 invertase is entirely absent, as was found to be the case by Robertson, Irvine and 

 Dobson. 



Journ. of Agric. Sei. vn 21 



