360 Carhohyclrates of the Leaf of the Potato 



predominating constituents of the sap in the stalks, and could very well 

 be translocated directly as such from the leaf. It is possible, and may 

 be argued, that the saccharose in the leaf serves to regulate the osmotic 

 pressure, owing to the ready interconversion of saccharose and hexoses ; 

 but in plants which form starch, such as the potato, this regulation 

 could be quite as well effected by the precipitation of the polysaccharide 

 and the function of the cane sugar is not easily understood unless it be 

 regarded as a primary and compulsory product of the mesophyll. 



The Dexirose-Laevulose Ratio. As was the case in the mangold 

 leaf, it is shown that it is impossible to obtain accurate values for 

 dextrose and laevulose owing to the presence in the solutions of 

 optically active impurities which are not removed by the ordinary 

 process of defecation by basic lead acetate. These impurities also 

 interfere with the estimation of the saccharose by the double polarisation 

 method and, as in the mangold, the fluctuations of the apparent dextrose 

 and laevulose can be correlated with the divergences between the 

 values found for saccharose by the reduction and by the optical methods 

 (see p. 344). It appears that two optically active impurities with 

 rotations of opposite sign are formed at different periods of the 24 hours, 

 and it is the variation of these that causes the apparent fluctuations in 

 the proportion of dextrose and laevulose. In the leaf a substance 

 with a laevo-rotatory power generally predominates, so that the laevu- 

 lose appears to be greatly in excess of the dextrose; but in the stalks 

 this is no longer the case and dextrose appears to be largely in excess 

 of the laevulose. 



Experimental. 



The methods of sampling, extraction, and analysis were the same 

 as those described in the case of the mangold (see Paper I). The potatoes 

 (King Edward VII) were grown on a piece of ground at the side of the 

 laboratory; at the date of picking (July 16th~17th, 1914) the plants 

 were just beginning to form flower buds and the tubers were small. 

 Rain had fallen heavily on July 12th, but the days following were dry 

 and sunny. Pickings were taken every two hours. The leaflets were 

 detached from the rachis, but the mid-ribs of these leaflets were not 

 cut out so that the results given for "leaves" refer to the whole leaflets 

 including these mid-ribs; what we have called "stalk" consisted in 

 reality mainly of the rachis of the compound leaves and included only 

 a small portion of the main stalk or stem, namely the portion furthest 

 from the tubers. 



