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 



