212 HOW CROPS FEED. 
The application of these principles to the question be- 
fore us is apparent. The young, active rvots that are in 
contact with the soil are eminently hygroscopic, as is de- 
moustrated by the fact that they supply the plant with 
large quantities of water when the soil is so dry that it 
has no visible moisture. They therefore share with the 
soil the moisture which the latter contains. As water 
evaporates from the surface of the foliage, its place is 
supplied by the adjacent portions, and thus motion is es- 
tablished within the plant which propagates itself to the 
roots and through these to the soil. 
Each particle of water that flies off in vapor from the 
leaf makes room for the entrance of a particle at the root. 
If the soil and air have a surplus of water, the plant will 
contain more; if the soil and air be dry, it will contain 
less. Within certain narrow limits the supply and waste 
may vary without detriment to the plant, but when the 
loss goes on more rapidly than the supply can be kept up, 
or when the absolute content of water in the soil is re- 
duced to a certain point, the plant shortly wilts. Even 
then its content of water is many times greater than that 
of the soil. The living tobacco plant cannot contain less 
than 80°|, of water, while the soils in Sachs’ experiments 
contained but 12.3°|, and 1.5°|, respectively. When fully 
air-dry, vegetable matter retains 13°|, to 15°|, of water, 
while the soil similarly dry rarely contains more than 
1-2"|,. 
The plant therefore, especially when living, is much 
more hygroscopic than the soil. 
If roots are so hygroscopic, why, it may be asked, do 
they not directly absorb vapor of water from the air of 
the soil? It cannot be denied that both the roots and fo- 
liage of plants are capable of this kind of absorption, 
and that it is taking place constantly in case of the roots. 
The experiments before described prove, however, that 
the higher orders of plants absorb very l.téle in this way, 
