380 Carhohjidrates of the Leaf of the Potato 



mangold stalks is that in the former the curves of apparent dextrose 

 and apparent laevulose run almost parallel to one another throughout 

 the 24 hours, and at the same time about parallel to the saccharose 

 curve (see Fig. 2, p. 371). Each sugar increases slightly and continuously 

 during the day and then falls at night. 



It seems probable from the parallelism of the curves of saccharose 

 and total hexoses that the dextrose and laevulose are actually present 

 in the stalks as invert sugar, being formed from the saccharose by 

 inversion; the large apparent excess of dextrose would then be due 

 to the presence of a dext ro-rota,tory impurity which accumulates in the 

 stalks (whereas in the leaf a Zaero-rotatory substance is generally in 

 excess). The divergence A between the reduction and polarisation 

 values is relatively small in the case of the potato because the substance 

 is of such a nature that the change of rotation brought about by the 

 processes of inversion is relatively small; but the existence of this 

 divergence (up to 15 per cent.) is a proof that some such compound is 

 present. On the other hand the practical parallelism of the curves of 

 apparent dextrose and laevulose, which is in striking contrast with the 

 mangold stalks, suggests that whatever be the impurity which is present, 

 its amount remains relatively constant throughout the 24 hours. 



On the other hand the relatively small divergence between the 

 polarisation and reduction values for saccharose in the potato stalks, 

 in contrast with the large divergences found in the mangold stalks, 

 may be taken to mean that only small amounts of the optically active 

 impurity are present in the potato stalks and that the values of dextrose 

 and laevulose in the stalks {not in the leaves) nearly represent the true 

 values for these sugars. If this is the case the dextrose has accumulated 

 in the stalks far more than the laevulose, possibly owing to the latter 

 sugar being used up for tissue building', and to the fact that the 

 starch formed in the leaf gives dextrose as sole product when hydrolysed 

 by the leaf enzymes (see p. 3.57). One would naturally expect the starch 

 in the tuber to be built up from dextrose as it yields dextrose exclusively 

 when hydrolysed by either acids or taka-diastase and the predominance 

 of dextrose in the stalks conveying sugars to the tuber would be quite 

 natural if this were the case. The question, however, whether the 

 dextrose is in actual excess over the laevulose in the stalks or whether 



' It is interesting to recall Meyer's observation in ISSO that almost all leaves capable 

 of forming starch at all produce it abundantly from a 10 per cent, solution of laevulose 

 and a relatively small number only from dextrose. On general grounds, considering the 

 relationship of starch and dextrose, one would have expected the reverse to be the case. 



