Lhyury from Cold During Growing Period. 121 
_ B— MetHonps oF AVERTING INJURY FROM CoLD DURING THE 
GROWING PERIOD a 
205. Plants are much more susceptible to injury from 
cold during their growth period than during their dormant 
period (171). Comparatively few plants, however, are in- 
jured bv cold in any season until the temperature falls be- 
low the freezing point of water (32° F., O°C.), 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 in the least compressed unless a por- 
tion of the water escapes. If itis 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 this, that it is 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. . 
Now 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 
will have to be set free. If the air were only half saturated 
at 50° F., its temperature could be reduced considerably 
before any of its water would have to be set free; but when 
a certain degree of reduction is reached, the air will no 
longer be able to hold all its water, and a part will be set 
Tree, gor in other words, will be precipitated. The cooling of 
