PLANT PHYSIOLOGY. 93 



dry tissues of the winter buds and ripened stems of the na- 

 tive trees and shrubs in cold countries are rarely injured 

 even in the severest winters, while the young leaves and 

 shoots in the spring are often killed by slight frosts. 



165. Death from low temperature is always accompanied 

 by the formation of ice-crystals in the succulent tissues; 

 these are formed from the water of the plant, which is 

 abstracted from it in the process of congelation. Much of 

 the water thus frozen is that which fills the cavities (vacu- 

 oles) of the cells, while some of it is that which moistens 

 the protoplasm and cell-walls. 



166. As the liquid in the vacuoles is not pure water, but 

 a mixture of several solutions, it freezes at a lower tem- 

 perature than water, and then, according to a well-known 

 law of physics, separates into pure ice-crystals and a denser 

 unfrozen solution. By a greater reduction of temperature 

 more ice-crystals may be separated out and the remaining 

 solution made denser still. This increasing density tends 

 to retard the formation of ice-crystals, and it is probable 

 that it is only in extremely low temperatures, if at all, that 

 the liquids in the plant are completely solidified. 



167. A plant which has been frozen may survive in many 

 instances if thawed slowly, but if thawed quickly its vitality 

 is generally destroyed. Thus many herbaceous plants will 

 endure quite severe freezing if they are afterward covered 

 so as to secure a slow rise of the temperature, and many 

 bulbs, tubers, and roots will survive the severest winters if 

 covered deeply enough to prevent sudden thawing. Like- 

 wise turgid tissues, which are not living, as those of many 

 succulent fruits, are injured, or not, by freezing according 

 as the thawing has been rapid or slow. 



Practical Studies. — (a) Plant a few seeds of radish, barley, wheat, 



