128 TEXTBOOK OF PLANT PHYSIOLOGY 



the plant. A similar accumulation of sugar may be observed in 

 potatoes J stored at a temperature about 0° C, and the increase of 

 sugars is readily perceptible to the taste. This unpleasant flavor 

 is easily removed by keeping such sweetish potatoes for some time 

 in a warm room. 



Hardening, however, is not entirely due to the accumulation of 

 protective sugar, the protein substances of the protoplasm also 

 undergo changes, although these are not quite clear to us. It is 

 known that irreversible coagulation sets in only at considerably 

 lower temperature after hardening than before it. Especially 

 marked seasonal variations in hardiness may be observed in trees. 

 The needles of conifers, for instance, are able to withstand winter 

 temperature of —30 to —40° C. while in summer they perish at 

 -8°C. 



Besides the accumulation of protective substances and the 

 changes in the colloids of the protoplasm, increased hardiness is 

 attained also by reducing the amount of free water. Dry seeds, 

 containing not more than 10 to 12 per cent of moisture, are not sus- 

 ceptible to frost. They can stand without injury the temperature 

 of liquid air and even that of liquid hydrogen. Soaked seeds 

 perish at — 8 to — 10° C, and germinated seeds are still more 

 susceptible to frost. The mature and dormant wood of branches 

 of trees is much more frost resistant than the wood of growing 

 shoots. In such desiccated tissues only a small amount of ice 

 is formed even by severe freezing, hence they are little injured 

 by frost. 



In addition to the factors which increase frost resistance, there 

 are also others which decrease it as, for instance, the abundance of 

 free water or the acidity of the cells. Different tissues, and even 

 separate cells of the same tissue, may show different degrees of 

 frost resistance. In the seedlings of cereals, for instance, the most 

 susceptible part is the plumule and the hardiest are the root tips 

 and their central cylinders, while cortical parenchyma of the root 

 is comparatively little frost resistant. After moderate freezing 

 the root tips, therefore, continue to grow for some time while the 

 young leaves and the external portions of the roots are killed. 



Attempts to increase frost resistance artificially have not led 

 so far to any positive results. The principal means of controlling 



1 But in this case, the ability to survive after freezing is not acquired. 



