9 8 BOTANY OF THE LIVING PLANT 



evaporation. About 60 to 70 per cent, of the volume of an arable soil is 

 made up of soil-constituents as above detailed, and about 30 to 40 per cent, 

 is accounted for by spaces between the soil particles, occupied by air (which 

 may be somewhat different in composition from atmospheric air) and by water. 

 In badly drained soils the spaces are largely occupied by water, and aeration 

 is correspondingly reduced. The soil-water carries in solution small quantities 

 of any constituent of the soil that is soluble. Examples of materials commonly 

 found in solution in the soil are mentioned above. 



Besides the plants rooted in it, the soil houses a vast population of other 

 organisms. Earthworms are constantly at work burrowing through the soil, 

 passing large volumes of it through their alimentary canal, and voiding it 

 at the surface as worm-castings. They bring up material from the lower 

 layers to the surface, and conversely they draw down into their burrows 

 leaves and other decaying parts, thus serving as tillers of the soil. Many 

 other animals are also present, such as wire- worms, eel- worms, etc. There 

 are also multitudes of microscopic forms of life, such as the Protozoa, Fungi 

 and Bacteria. The latter play a very important role in the decay of organic 

 materials in the soil, and in other ways by the varied chemical changes which 

 they bring about (see Chapters VIII. and XXVIII.). 



The root-hair in exercising its function of absorbing water has to 

 face a certain resistance from the soil. This arises from the physical 

 forces of capillarity and imbibition, and from the slight osmotic 

 activity which the soil-water, with its salts in solution, possesses. 

 The magnitude of this last factor is unusually high in salt-marshes 

 and other sea-side situations, where the salt-content of the soil may 

 be considerable. But the fact that plants usually manage to keep 

 that condition of turgor, on which their firmness and mechanical 

 rigidity largely depend, indicates that the absorptive forces of root- 

 hairs are adequate to overcome these resistances to a requisite extent 

 (see Chapter X.). It is probable that plants utilise chiefly the capil- 

 lary water, since the colloidal particles of soil cling very tenaciously 

 to their imbibed water, and the absorptive forces of the root-hairs 

 may be inadequate to overcome these forces of imbibition. Reference 

 has been made to the osmotic pressure of the root-hairs. The actual 

 proportion of this osmotic force that is available for absorption at 

 any moment depends on the degree to which the root-hairs are 

 removed from complete turgor. It was pointed out in Chapter III. 

 how a fully turgid cell has no absorptive capacity (or Suction Pressure), 

 while this capacity is at its maximum in a plasmolysed cell. Only 

 if there is a steady movement of water out of the root-hairs into 

 the interior of the root will absorption by the root-hairs continue, for 

 otherwise complete turgor would soon be reached and absorption 

 cease. 



