114 THE CARBOHYDRATES 



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 formed, and as the water is thus withdrawn, the sugar becomes 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

 Finally, as Parkin points out, the nature of the carbohydrate 

 reserve may depend on the kind of sugar transformed ; thus, 

 if saccharose be the chief sugar translocated from the leaves, 

 then it might be expected that starch would be produced, on 

 the other hand inulin might be formed if the available sugar 

 for conversion were levulose. 



Preparation. 



Inulin may be obtained from dahlia tubers, of which it 

 forms from 10-12 per cent, by crushing them and pressing out 

 the liquid ; the residue is then boiled up with a little water 

 and some precipitated chalk and filtered again. The two 

 filtrates are then united and once more boiled with chalk in 

 order to neutralize any acids, and while still warm treated 

 with lead acetate until no further precipitate is formed. The 

 filtered solution is then saturated with hydrogen sulphide, fil- 



