178 THE CARBOHYDRATES 



proteins. Parkin points out that the presence of inuUn * 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-forma- 

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

 substances, 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 formed, and as the water is thus withdrawn, the sugar becomes 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 sufhcient 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 Dicraniim which contains 

 much oil is less resistant to cold that 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 oil-containing seeds of the linseed are highly 

 resistant. Such divergent phenomena must depend on the 

 constitution of the protoplasm. 



Again, oil is a convenient form of reserve food, especially 

 in small organisms and in reproductive bodies, where space is 

 limited and lightness is all-important and it is desirable to 

 store a maximum of potential energy in the minimum of bulk. 



INULIN-LIKE SUBSTANCES. 



A number of ill-defined substances similar to inulin have 

 been described as occurring in various plants. The chief of 

 these are : — 



Graminin in Agrostis, Fesluca, Triticum. Arrhenatherum, 

 and other grasses. 



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



