PHYSIOLOGY 199 



HAIRS, delicate tubular outgrowths of the epidermal cells. Although 

 they have the diameter of only a medium-sized cell, and appear to 

 the naked eye as fine, scarcely visible, glistening lines, they often 

 attain a length of several millimetres and greatly enlarge the ab- 

 sorbing surface of their parent root. According to F. SCHWARZ the 

 epidermal surface of the piliferous zone of the roots of Pisum, which has 

 230 root-hairs to the square millimetre, is thus increased twelvefold. 



The root-hairs cover only a comparatively small zone, a short 

 distance above the growing root-tip. Soon after they have attained 

 their greatest length, and have come into the closest contact with the 

 particles of the soil, they die off. Above this advancing zone of hairs 

 the epidermis of the root becomes again completely divested of root- 

 hairs (Fig. 186). The older parts of roots take no share in the process 

 of absorption. They envelop themselves with cork, increase their con- 

 ducting 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 con- 

 fined to the regions covered with root-hairs, or, in case no root-hairs 

 are developed, to a corresponding 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 coarse sand to li pe.r cent. Plants may 

 even obtain a certain quantity of water from soil which is frozen 

 hard or from a block of ice. 



The ABSORPTIVE POWEU 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 mainly take place in soils rich in clay, 

 lime, or humus, and containing double silicates of alumina. Salts of potassium 

 and ammonium, also those of calcium and magnesium, and phosphates are absorbed 

 by these soils. The former form silicates or double silicates that are only with 

 diliiculty dissolved, while the phosphoric acid is held combined with calcium or 

 iron. Magnesium and calcium salts are in other soils 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 (cf. p. 189). 



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. 



