W. A. Davis and G. C. Sawyer 881 



the two sugars are present mainly in the form of invert sugar can only 

 be decided definitely when methods have been devised of estimating 

 the two sugars, in presence of each other, which are free from the errors 

 caused by optically active impurities. 



Summary. 



1. In the potato leaf when the tubers are beginning to develop the 

 principal sugar present is saccharose ; its amount increases from sun- 

 rise up to 2 p.m., following approximately the curve of temperature. 

 It then falls during the rest of the day and night. The rise and fall are 

 both linear. 



2. The hexoses are present in the leaf in very small amounts — 

 generally less than 1 per cent, of the total dry weight of the leaf. 

 They fluctuate considerably during the early part of the day, the 

 fluctuations being apparently determined by conversion into or 

 formation from starch. 



3. During the early part of the day up to 2 p.m. the proportion 

 of starch changes very little, the small fluctuations which occur being 

 related to changes in the starch. The starch is apparently formed 

 from the hexoses. 



i. Directly the amount of saccharose has reached its maximum 

 at 2 p.m. the hexoses begin to increase in the leaf, owing apparently 

 to hydrolysis of the saccharose to invert sugar; at the same time 

 " soluble starch " (or dextrin) is first detected in the leaf and its amount 

 increases regularly up to 6 p.m. At 6 p.m., 2 hours before sunset, the 

 true starch in the leaf reaches a maximum value, far greater than any 

 previous value during the day. The starch and "soluble starch" 

 subsequently fall rapidly until between midnight and 2 a.m. the amount 

 left is exceedingly small (0-2 per cent.). The starch is apjjarently con- 

 verted directly into hexose (dextrose), the amount of which increases 

 in the leaf. 



5. In the stalks reducing sugars predominate greatly over the 

 saccharose in spite of the fact that in the leaf the latter is in excess. 

 As in the mangold it is probable that cane sugar is the first sugar formed 

 in the leaf and that it is hydrolysed by invertase in the veins, mid-ribs 

 and stalks, for the purpose of translocation. 



6. As in the mangold, the true proportions of dextrose and laevulose 

 cannot be determined in the leaves and stalks owing to the presence of 

 soluble optically-active impurities, which vitiate the polarimetric data. 



