W. A. Davis 331 



variation in the amount of the optically active impurities which are 

 present; if the latter, the fluctuation of these substances during the 

 24 hours must be quite as great as the fluctuation of the sugars themselves. 

 Until a method has been devised by which it is possible to estimate 

 accurately the true proportions of dextrose and laevulose, when 

 present together, without having recourse to polarimetric data, the 

 facts brought forward in this paper, and that which follows, show that 

 it is impossible to know with any certainty the real proportions of these 

 sugars present in different jDlant tissues; it is, therefore, ec^ually 

 impossible to draw conclusions as to the function of these two sugars 

 in the plant — whether the one is more suited than the other to build 

 up new tissue or whether one is more easily put under contribution 

 than the other in respiration. The fermentation test, which Parkin 

 and others have used to ascertain whether the solutions they analysed 

 were free from optically active substances other than sugars, is one 

 which is by no means reliable for this purpose. Parkin, whose work 

 in most other respects is valuable, considered that, as the solutions 

 prepared from the snowdrop leaves showed, after fermentation with 

 yeast, a negligible rotatory and reducing power, no other substances 

 likelij to possess these properties were present in the original solutions. It 

 would be quite possible for large amounts of asj)aragine to have been 

 present, sufficiently large indeed to explain the apparent preponderance 

 of laevulose in the snowdrop (where the ratio of laevulose to dextrose 

 varied from 1 : 0-4 to 1 : 0-76) and yet to have escaped detection by 

 this method, as the asparagine would be largely, if not entirely, consumed 

 by the yeast in its growth ; asparagine indeed is used very largely as 

 a nutrient material for yeasts, for example in Hayduck's solution. 



Experimental. 



The methods of analysis have been described in the preceding paper ; 

 an example is there given of the method of calculating the proportions 

 of dextrose and laevulose according as the pentoses are assumed to be 

 arabinose or xylose (see p. 318). The actual data used are given on 

 pp. 319-325 ; the results are calculated on the total vacuum-dried ^natter 

 of the leaf. 



In the tables which follow, D = per cent, of "apparent dextrose" 

 in the total vacuum-dried matter ; L = per cent, of " apparent laevu- 

 lose" in the total vacuum-dried matter. 



