52 THE SYNTHESIS OF CARBOHYDRATES 



found pentose to be present whilst maltose was consistently 

 absent, both by day and by night, from the leaves mentioned. 

 In view of the presence of starch in the potato leaf, it is sur- 

 prising that the presence of maltose was not established : 

 there is, however, good reason for supposing that maltase 

 is commonly present in plants in which starch occurs and 

 is digested : this enzyme is rather indiffusible, is destroyed 

 by alcohol and chloroform in ordinary conditions, and is 

 very susceptible to heat, a temperature of • 50 C. having a 

 marked adverse effect upon its activity, whilst at 55 C. it 

 may be destroyed. It is these properties of maltase which 

 account for the varying statements regarding the occurrence 

 of maltose in green leaves : if the procedure of experiment is 

 such as to destroy the maltase, maltose will be found ; if, on 

 the other hand, the enzyme is not destroyed, the maltose is 

 converted into hexose sugars. * Davis, Daish, and Sawyer 

 dropped their material of experiment into boiling alcohol and 

 thus destroyed any maltase that might have been present ; 

 their inability to detect maltose was therefore not due to the 

 activity of maltase. The conclusions of these authors regard- 

 ing the presence of pentose — they estimated that pentose 

 represented from 5 to 25 per cent, of the total sugars — is ad- 

 versely criticized by Colin and Franquet f who found that the 

 amount of this sugar in the leaves of the potato and of the 

 beetroot is at most less than 0-03 per cent, of the total sugars. 

 Gast J examined the carbohydrate content of the leaves of 

 Tropceolum, Vitis, Musa and other plants collected in the early 

 afternoon and early morning : he found that sucrose always 

 was more abundant than maltose and the other sugars identi- 

 fied ; he is in accordance with Brown and Morris regarding 

 the absence of pentose. Kylin § found that in general terms 

 the amount of glucose varies inversely as the amount of starch. 

 Sucrose is generally the most abundant sugar, especially in 

 sugar leaves, but Gentiana brevidens, a sugar leaf, does not 



* Davis : " Biochem. Journ.," 1916, 10, 31, 49, 56. 



f Colin and Franquet : " Bull. Soc. chim. biol.," 1927, 9, 114. 



X Gast : " Zeit. physiol. Chem.," 1917, 99, 1. 



§ Kylin : id., 1918, 101, 77. 



