HEART-ROT 119 
The discussion must thus be carried back a step farther, 
viz. to a consideration of the agencies which facilitate the 
aeration of the subsoil in the cases of cultivated land and 
woodland. 
These. agencies are notably divergent in the two .cases. 
In arable land the surface soil is very thoroughly aerated 
to a depth of 5 to 7 in. by ploughing, and somewhat more 
where a steam plough is used, but below this there is very 
little disturbance of the soil particles. Three factors may, 
however, assist, though slightly, in bringing fresh supplies 
of oxygen to the subsoil. These are—(1) worms, which 
burrow deep in cold winters and in dry summers (a depth 
of 5 ft. is recorded as not uncormmon in Darwin’s Vegetable 
Mould and Earthworms, p. 112) ; (2) rain, which percolates 
through to the subsoil carrying with it a small amount of 
dissolved oxygen, and allowing room for more air when it 
drains away ; and (3) frost, which very rarely affects the 
subsoil in our climate. The combined action of these three 
factors is very slight in most soils, and only in those with 
very deep drainage is the subsoil at all effectively aerated. 
In woods, worms are comparatively infrequent, and frost 
is entirely negligible. But there is a new factor which is 
incomparably more efficient than any in arable land, viz. 
roots, which act in a multiplicity of ways. By boring their 
way into the subsoil and by their secondary growth in 
thickness they force apart the soil particles, and if they 
die and rot the space which the root formerly occupied 
‘must become filled again. Also the organic remains will 
improve the soil. Still more important than all these 
mechanical disturbances is the absorption of water by the 
roots. For when roots draw in water from between the 
particles immediately surrounding them the equilibrium 
in the soil is disturbed and water is attracted by capillarity 
from the more distant, moister soil into that which has 
been dried through its propinquity to the roots. This 
system by which roots obtain water from a distance is 
depicted in the well-known diagram in Sachs’s Physiology 
of Plants, which has been copied into most subsequent text- 
