72 CARBOHYDKATES [ch. 



As to the question of which sugars are present in the leaf, there is 

 only evidence from accurate work on a few plants. Careful investiga- 

 tions have been made of the sugars in leaves of the Mangold (Beta 

 vulgaris) (Davis, Daish and Sawyer, 17), Garden Nasturtium (Tropaeo- 

 lum majus) (Brown and Morris, 6), the Snowdrop (Galanthus nivalis) 

 (Parkin, 26), the Potato (Solanum tuberosum) (Davis and Sawyer, 19) 

 and the Vine (Vitis vinifera). The general conclusions drawn from 

 these investigations are that sucrose, glucose, and laevulose are always 

 present in leaves: that maltose results from the hydrolysis of starch, 

 being absent from leaves which do not form starch. Maltose is not pre- 

 sent in appreciable quantity even in starch-producing leaves because it is 

 rapidly hydrolyzed into glucose by maltase. (In such cases where it has 

 been detected it has been due to diastase action during the drying of 

 leaves before extraction.) Other leaf carbohydrates are the pentoses 

 which have been found in a good many species examined and may be 

 widely distributed; the pentosans, their condensation products, also occur 

 as well as dextrin (Potato). 



The next question to be considered is what sugar is first synthesized 

 in the leaf. Is it glucose, laevulose, sucrose or maltose ? It is known that 

 the enzymes, invertase and maltase, are commonly present in leaves and 

 that these enzymes respectively control the hydrolysis, of cane-sugar 

 into glucose and laevulose, and of maltose into glucose. It is also possible 

 that they respectively control the synthesis of sucrose and maltose. 

 Laevulose, likewise, as may be supposed, can be obtained from glucose. 

 Thus all the sugars can be readily converted one into another, but to 

 ascertain which is the first product of synthesis is not an easy problem. 



In addition to the above-mentioned work on the nature of the sugars 

 present in leaves, a good deal of careful analysis has been made as to the 

 proportions in which the sugars occur relatively to each other during 

 stated periods of time, with a view to answering the question as to 

 which is the first-formed sugar. There are two possibilities: one, that 

 it is sucrose and that it is hydrolyzed into glucose and fructose: the 

 other, that it is glucose, from which fiructose is derived, and the two are 

 then synthesized to form sucrose. 



Opinion is divided on this point and there is not at present sufficient 

 experimental evidence to decide the matter. The majority of investi- 

 gators regard sucrose as the first-formed sugar, and suggest that it is 

 inverted into hexoses for purposes of translocation, since the smaller 

 molecules would diffuse faster. There is experimental evidence that 

 there is an increase in hexoses in the conducting tissues. Others favour 



