206 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



THE SUGARS OF THE TRANSPIRATION STREAM. 

 A survey of the roughly quantitative estimations of the 

 sugars as recorded in the foregoing tables makes it clear 

 that, though hexoses and maltose may be present, sucrose 

 is by far the most important sugar of transport. Thus, 

 in Populus, Acer, Cotoneaster, and Salix, it far exceeds 

 the hexoses and maltose combined. In Ilex and Fagus 

 the proportions appear to be more evenly balanced, 

 though on two occasions sucrose was found to be entirely 

 absent from the wood of the latter. This preponderance 

 of sucrose is decidedly remarkable. Its presence in such 

 quantity cannot, it seems, be explained by supposing it 

 to have been stored as such in the parenchymatous cells of 

 the wood, for these are loaded with starch in quantities 

 sufficient to produce maltose and glucose in amounts far 

 in excess of the sucrose found; yet the proportion of the 

 latter preponderates. Since maltose, by the action of 

 maltase, gives rise to glucose only, one is led to suppose 

 that a part of this hexose undergoes transformation into 

 d-fructose (laevulose), and that these two are then united 

 to form sucrose. The possibility of such changes taking 

 place has been discussed in Chapter I. The alternative, 

 that sucrose is here directly derived from starch, just as 

 starch is condensed from sucrose, must also be considered. 



In this connection it is of interest to note that Hassel- 

 bring and Hawkins (1915) have shown that, when the roots 

 of the sweet-potato are stored, the starch present in them 

 is rapidly transformed into sucrose and hexose, of which 

 the former is in large excess. These authors give a 

 bibliography of earlier work on similar changes in other 

 plants. Gore (1914) has also shown that sucrose increases 

 as starch decreases in ripening bananas. 



The very general absence of any appreciable quantities 



