LEVULOSANES 133 



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 tha sugar that remains some has been used 

 up in respiration is reconverted into starch. 



(Ecologically these characters are of value to the plant ; for if 

 the water of the cell sap be frozen, the salts held in solution be- 

 come 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 evapora- 

 tion 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- 

 mized. But, notwithstanding these facts, plants are frequently 

 subjected to temperatures sufficiently low to cause ice to be 

 formed, and as the water is thus withdrawn, the sugar becomes 

 more concentrated until it will also crystallize out. Both 

 these processes generate heat, which may be sufficient in 

 amount to enable the protoplasm to live. And this is, accord- 

 ing to Mez and Lidforss, the explanation of the presence of 

 sugar in winter leaves. 



At the same time we must be careful not to push such 

 explanations too far, for there are many exceptional cases; 

 thus Ewart has pointed out that Dicranum which contains 

 much oil is less resistant than is Bryum, and other mosses, in 

 which such substances are absent. The beetroot also is very 

 susceptible to cold, notwithstanding the fact that it contains 

 much sugar; similarly the seeds of the hemp and willow, 

 which* contain much oil, are easily killed by desiccation, 

 whereas the qil-containing seeds of the linseed are highly 

 resistant. Such divergent phenomena must depend on the 

 constitution of the protoplasm. 



* See also Grafe and Vouk: " Biochem. Zeitsch.," 1913, 56, 249. 



