THE ROOT 81 
the root shows that here again respiration is taking 
place. Carbon is here combining with oxygen to supply 
the energy needed by the root to do its work. Carbonie 
acid is not the only acid given off by the root. From 
the fact that respiration takes place in the root, it may 
be inferred that oxygen is needed by that organ, and 
simple observations go to show that such is the ease. 
Where the roots of plants are in water-logged soil, or 
where the soil is so compressed as not to admit of the 
free ingress of air, they become stunted and unhealthy, 
and the whole plant suffers. This ineidentally draws 
attention to the fact, that, however necessary water 
may be, a soil may contain too much water; and, 
further, emphasizes the need for working a soil to 
loosen it and admit the air. 
Water is held in the soil in three ways, first as free 
or gravitational water, which has sunk deep into the 
lower regions of the soil and there forms the chief 
reservoir of supply for trees and other deep-rooting 
plants. There is then the capillary water which is lifted 
from the reservoir below by the tube-like passages that 
wind among the soil grains, and supplies the grass and 
other surface feeders. Lastly hygroscopic moisture 
forms an invisible film round the apparently dry soil 
erains, and probably resists all efforts of the root hairs 
to withdraw it from the soil. To regulate the supply 
of moisture and let air into the roots of plants it is 
often necessary to resort to drainage. This results in 
keeping the free or gravitational moisture from getting 
so near the surface as to drive out the air from those 
soil regions that are occupied by the roots of ordinary 
farm crops. 
NtTROGEN ABSORPTION. 
Nitrogen is of prime importance to the plant, for 
it is the essential constituent of the lfe substance 
F 
