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 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 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 



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