PHYSIOLOGY 183 



just above the tip, will be found clothed for a short distance with earth 

 particles held fast by root-hairs, which thus mark the zone occupied by 

 them. The older parts of roots, even in plants which persist for many 

 years, take no part in the process of absorption. They envelop them- 

 selves with cork, increase their conducting elements by growth in 

 thickness, and function exclusively in the transfer of the water 

 absorbed by the younger portion of the roots. Even in the young 

 roots the absorption seems principally confined to the regions covered 

 with root-hairs, or, in case no root-hairs are developed, to a correspond- 

 ing zone of the root epidermis. 



Through the intimate union of the youngest roots with the soil, 

 they are able to withdraw the minute quantity of water still adhering 

 to the particles of earth, even after it appears perfectly dry to the sight 

 and touch. There still remains, however, a certain percentage of 

 water, held fast in the soil, which the roots are not able to absorb. 

 Thus, Sachs found that the water left by a Tobacco plant, and which 

 it could not absorb, amounted in cultivated soil to 12 per cent, in 

 loam to 8 per cent, and in sand to 1J per cent. The root-hairs seem 

 to take up chiefly the substances held by the soil by means of its 



ABSORPTIVE POWER. 



The absorptive power of soil depends, partly, upon chemical changes taking 

 place within it, but partly also on physical processes (the superficial adhesive 

 force of its particles). The chemical changes are especially concerned with the 

 retention of ammonium and potassium salts, as well as phosphates ; the former as 

 difficultly soluble silicates or double silicates, while phosphoric acid is held in 

 combination with calcium or iron. Magnesium and calcium salts are, on the 

 contrary, but slightly absorbed. They are, like the chlorides, the nitrates, and, 

 in part, also the sulphates, easily displaced ; in soil treated with a solution of 

 saltpetre, for example, the potassium will remain in combination in the soil, while 

 calcium nitrate passes off in solution. 



Humus acids contribute, to a certain extent, to the chemical changes occurring 

 in soil, as do also soil bacteria, which possess strongly oxidising and reducing powers. 



The absorptivity of the soil, which, moreover, is not absolute, and varies with 

 different soils (sandy soil absorbs poorly), operates advantageously for plants by 

 the consequent rapid accumulation of large supplies of food-material for their 

 gradual absorption. 



The absorptive power of soil for water is due to its capacity to retain water by 

 capillarity, so that it does not run off. Of the soils investigated by Sachs, 

 cultivated soil retained in this way 46 per cent, loam 52 per cent, and sand only 

 21 per cent of water. 



The activity of the roots in providing nourishment is not only 

 manifested in overcoming the adhesive and absorptive power of the 

 soil. The young roots, and especially the root-hairs, in addition to 

 the carbonic acid exhaled by them, and which, no doubt, also aids in 

 loosening the soil, excrete a stronger acid, by means of which they 

 dissolve otherwise insoluble substances. Eoots growing upon a 

 polished plate of marble will so corrode it that an etched pattern of 



