72 VEGETABLE PHYSIOLOGY 
on osmosis between contiguous cells, have not the definite 
direction of the rapid current, and play quite a subordinate 
part in the supply of the whole plant with water. They 
are, however, supplementary to the ascending sap, and effect 
interchanges in regions which the latter does not immediately 
reach. The cortex of the axis of the plant is especially 
dependent upon them, as various mechanisms exist in the 
different regions of the stele to guard against too free an 
escape of water from its tissues into the cortex. 
Except in some special cases the water which passes 
through the body of an ordinary terrestrial plant is obtained 
from the soil in which its roots are embedded. The soil 
itself is composed of minute particles of inorganic matter of 
very different degrees of solubility, derived originally from 
the breaking down of rocks, together with decaying animal 
or vegetable matter mixed with the inorganic constituents. 
This organic matter is known as humus and is of very varied 
composition. The soil thus consists of a loose matrix of 
granular character, the interspaces of which are normally 
filled with air. The air is in most cases mixed with a certain 
quantity of carbon dioxide which is being evolved from the 
humus constituents of the soil, and which is slowly exhaled 
from the surface. The interspaces are capable of con- 
taining varying quantities of water ; indeed the soil may be 
so saturated with it that they are all full. We find soils of 
all conditions in this respect, from the dry sands of deserts 
to the mud of bogs. The water may be held with greater 
or less tenacity, clays and sandy soils affording instances of 
two extremes in that particular. When the interspaces of 
the soil are filled with water, the plants which it is support- 
ing are very unfavourably placed for absorbing the liquid. 
By the excess of water their roots are deprived of the air 
which they need for purposes of respiration ; their structure 
does not enable the absorption of water to take place all over 
their surfaces, as their external cells are more or less cuti- 
cularised ; they are consequently hindered and not helped by 
the superfluity of liquid. When a soil is properly drained, its 
