THE TRANSPORT OF WATER IN THE PLANT 67 
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 origi- 
nally from the breaking down of rocks, together with decay- 
ing 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 cages 
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 containing 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 supporting 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 cuticular- 
ised ; they are consequently hindered and not helped by the 
superfluity of liquid. When a soil is properly drained, its 
