ATMOSPHERIC AIR AS THE FOOD OF PLANTS. 35 
Water then exists in the atmosphere during the periods 
of vegetable activity as gas or vapor,* and as liquid. In 
the former state it is almost perpetually rising into the air, 
while in the latter form it frequently falls again to the 
ground. It is thus in a continual transition, back and 
forth, from the earth to the sky, and from the sky to the 
earth. 
We have given the average quantity of water-vapor in 
the air at one per cent; but the amount is very variable, 
and is almost constantly fluctuating. It may range from 
less than one-half to two and a half or three per cent, ac- 
cording to temperature and other circumstunces. 
. When the air is damp, it is saturated with moisture, so 
that water is readily deposited upon cool objects. On the 
other hand, when dry, it is capable of taking up additional 
moisture, and thus facilitates evaporation. 
Is Atmospheric Water Absorbed by Plants ?—It has 
long been supposed that growing vegetation has the power 
to absorb vapor of water from the atmosphere by its 
foliage, as well as to imbibe the liquid water which in the 
form of rain and dew may come in contact with its leaves. 
Experiments which have been instituted for the purpose 
of ascertaining the exact state of this question have, how- 
ever, demonstrated that agricultural plants gather little or 
no water from these sources. 
The wilting of a plant results from the fact that the 
leaves suffer water to evaporate from them more rapidly 
than the roots can take it up. The speedy reviving of a 
wilted plant on the falling of a sudden rain or on the depo- 
sition of dew depends, not so much on the absorption by 
the foliage, of the water that gathers on it, as it does 
* While there is properly no essential difference between a gas and a vapor, 
the former term is commonly applied more especially to aériform bodies which 
are not readily brought to the liquid state, and the latter to those which are easily 
condensed to liquids or solide, 
