RESISTANCE OF PLANTS 355 



These changes consist chiefly in the increase of the water- 

 retaining power of the colloids. Owing to this property, during 

 freezing they give up with great difficulty the water contained in 

 them ; and even at very low temperatures, a considerable amount 

 of unfrozen firmly bound water remains in them. Besides, 

 partial hydrolysis transforms the protein substances of the plasma 

 from more compUcated but unstable forms into simpler com- 

 pounds that are not so easily dehydrated during freezing. In 

 this process, a part of the amino acids is split off and accumulates 

 in the cells. 



Increase in endurance of low temperature is produced not only 

 by the accumulation of protective substances and by changes of 

 the colloids of the protoplasm but also by a general decrease in 

 the amount of free water that may freeze. Dry seeds, containing 

 only about 10 to 12 per cent of hygroscopic water, are absolutely 

 unsusceptible to frost and can endure without injury the tem- 

 perature of liquid air or even of liquid hydrogen. Soaked seeds 

 are killed at 8 to 10°C., while seeds that have begun to germinate 

 and have absorbed water abundantly are still more sensitive to 

 cold. Likewise, wood of branches that have terminated growth, 

 and are ripe and low in water content, is much more resistant 

 to frost than the wood of growing shoots. In such previously 

 dehydrated tissues, only insignificant quantities of ice are formed 

 even during severe frosts, and this is why they are less susceptible 

 to cold. 



In the hardening of winter cereals, light plays an important 

 role. With the approach of winter, these plants are quite young, 

 tillering sometimes not having yet begun, and thus they do not 

 have at their disposal a large reserve of carbohydrates. But 

 the sugars formed by photosynthesis in winter contribute con- 

 siderably to an increase in their frost resistance (Tumanov, 1930). 

 Therefore, in regions where there are many clear sunny days in 

 autumn, winter cereals are more hardy than in regions where 

 cloudy days prevail. Likewise, in years with a clear dry autumn, 

 winter crops are better hardened than in years with a wet and 

 dark autumn. 



In addition to the factors that increase frost resistance, there 

 are also others that decrease it, as for instance the abundance 

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

 even certain cells of the same tissue, may show various degrees 



