ii6 THE CARBOHYDRATES 



the leaves of mycotrophic plants, which generally show a feeble 

 transpiration, seldom contain starch, its place being taken by 

 glucose. Lidforss also has shown that the winter green vege- 

 tation of Sweden is characterized by the absence of starch from 

 the leaves, the mesophyll, in its place, containing relatively 

 large quantities of sugar, and sometimes oil during the winter 

 months. In summer the leaves of these plants contain 

 starch, which, on the advent of winter, is converted into sugar, 

 from which starch is formed on the rise of temperature in the 

 spring. 



Then, again, it is not uncommon to find sugar stored in the 

 periderm of trees and in the leaves of evergreen plants during 

 the winter ; starch, however, may be found in the leaves of 

 evergreen trees during the cold season, its presence being due 

 to feeble photosynthesis. 



Reference may be made here to the well-known fact that 

 potatoes turn sweet on exposure to cold. This conversion 

 of starch into sugar is most active at o° C, and the action de- 

 creases with the rise in temperature, so that above 7° C. no 

 sugar is thus produced. Also if the tubers are suddenly sub- 

 jected to a temperature of - i° C, no sugar will be produced. 

 The amount of sugar formed is not great, its maximum being 

 about 3 per cent of the wet weight ; the limit of the process 

 depends on the concentration of sugar, and, as Czapek has 

 shown, the transformation of the starch may be prevented, on 

 a lowering of the temperature, if the concentration of sugar be 

 sufficient. If these sweet potatoes be exposed to a higher 

 temperature, all the sugar that remains — some of course has 

 been used up in respiration — is reconverted into starch. 



CEcologically these characters are of value to the plant ; 

 for if the water of the cellsap be frozen, the salts held in 

 solution become concentrated and will eventually precipitate 

 the soluble proteins. 



Parkin points out that the presence of inulin in the cell 

 sap of the parenchymatous tissues would retard the evaporation 

 of water. It is a well-known fact that water in the presence 

 of oil may be much over-cooled before ice-formation takes 

 place, and the freezing point of water in which other sub- 

 stances, e.g. sugar, are dissolved is depressed, and thus the 

 danger arising from the salting out of the proteins is mini- 



