126 Principles of Plant Culture. 



204. Wind-Breaks, i. e., plantings of trees intended to 

 break the force of prevailing winds, act beneficially in 

 lessening damage from cold, in so far as they prevent 

 snow from drifting off the soil and mitigate the effects 

 of drying winds (190 c). 



B METHODS OF AVERTING INJURY FROM COLD DURING 

 THE GROWING PERIOD 



205. Plants are much more susceptible to injury from 

 cold during their growth period than during their dor- 

 mant period (171). Comparatively few plants, however, 

 are injured by cold at any season until the temperature 

 falls below the freezing point of water (32 F., OC.), 

 or when so-called hoarfrost occurs. It is this extreme 

 that we have chiefly to fear and to guard against during 

 the growing period. 



206. The Cause of Hoarfrost. A sponge saturated with 

 water cannot be compressed in the least unless a portion 

 of the water escapes. If it is but half saturated, it may 

 be compressed somewhat without any escape of the liquid, 

 but if the compression passes a certain limit, the water 

 will begin to escape. 



The air is like a sponge in being capable of taking up 

 a certain amount of water. But the amount of water the 

 air can take up depends very much upon its temperature, 

 its capacity for water increasing as the temperature rises, 

 and decreasing as it falls. 



Suppose a given amount of air at a temperature of 50 

 F. has taken up all the water it can hold at that tem- 

 perature. It is clear from what has just been said, that 

 if the temperature of this air is reduced, some of its water 



