450 THE BIOLOGY OF FLOWERING PLANTS 



the starch of the leaves is in winter converted into sugar, 

 and by this means the protoplasm is protected from freezing. 

 Maximow (19 12) has given experimental demonstration of 

 the fact that in the presence of electrolytes and non- electro- 

 lytes the temperature at vi^hich death from freezing takes 

 place is lovv^ered, and to a much greater extent than can be 

 accounted for by the actual lovi^ering of the freezing-point. 

 It seems this is rather connected with the eutectic point 

 of the solution, at which all the water and solutes freeze out, 

 and this indicates that death from freezing is due to the 

 complete withdrawal of water from the protoplasm, and the 

 consequent coagulation of the proteins. Solutes in the cell 

 sap protect the protoplasm by lowering the temperature at 

 which this coagulation sets in. The conversion of starch 

 into sugar by the action of cold is familiar in the sweetening 

 of potatoes exposed to frost. It also occurs in the cortical 

 cells of many trees, such as the oak, the elm, and the beech ; 

 in other trees, e.g. the conifers and the birch, the starch is 

 converted into oil, an emulsion of which is also supposed 

 to have a protective effect against frost. Lewis and Tuttle 

 (1920) have shown that the sugar content of evergreen 

 leaves increases markedly in the winter months. There is 

 no doubt that this chemical protection by sugars and salts 

 in the cell sap is very much more effective than the possession 

 of thick corky bark or of bud-scales ; these can do little to 

 prevent the fall of temperature in the underlying tissues 

 during spells of frost. 



Bud Protection. — Many perennial herbs die down in 

 winter and pass the cold months well protected underground, 

 under the litter of dead leaves, or under the covering of 

 snow. The parts which most require protection are the 

 buds, and such herbs present a striking contrast to deciduous 

 trees and shrubs vnt\v their buds fully exposed to the air, 

 though individually provided with tough bud scales. Bud 

 scales, as we have seen, must be chiefly effective against 

 drought and mechanical damage, the resistance to cold being 

 attained by internal means. The various modes of pro- 

 tection of the buds have been used by Raunkiaer (19 10, also 



