EFFECTS OF COLD 87 



with no resulting damage. From this last 

 series of tests, it may be safel\^ said that the 

 protoplasm of plants when in the proper rest- 

 ing stage is practically indestructible by cold. 

 Not only have seeds great power of resistance 

 to cold, but they are capable of carrying on 

 growth at low temperatures. The seeds of 

 several common cereals will germinate on 

 blocks of ice at a temperature of one or even 

 two degrees below the freezing point. 



If frozen leaves are taken in the hand and Appearance 

 crushed or bent, they retain the form given of frozen 

 them. During the crushing, the breaking of P^^"^*^ 

 the ice can be heard distinctly. Frozen plants 

 do not regain their elasticity upon thawing. 

 Upon the contrary, they become limp, partly 

 transparent, and exhibit changes of color 

 chiefly due to the destruction of the chloro- 

 phyll. The entire body of the plant has lost 

 its consistency, and the different tissues may 

 be easily stripped apart. When exposed to 

 the sun, the leaves shrivel and assume a rusty 

 brown or black color. They entirely resemble 

 charred leaves, and the farmer says that the 

 frost has "burned them." 



It is not to be taken for granted that all of 

 the cells of a plant are equally resistant to 

 cold. Thus it is known that hairs and the 

 guard cells of stomata remainactive when the 



