68 



VEGETABLE PHYSIOLOGY 



interspaces are filled with air, and a delicate film of water 

 surrounds each of its particles and adheres closely to it. 

 This water, often spoken of as hygroscopic water, is the 

 source of the plant's supply. The presence of air in the 

 interspaces supplies the wants of the root and frees it from 

 the difficulties which have been pointed out. 



The hygroscopic water adheres so closely to the 

 particles of the soil that it escapes ordinary observation ; 

 when, however, soil that has been allowed to dry at any 

 ordinary temperature till its interspaces are apparently 

 empty, is exposed to a heat approaching that of boiling 

 water, a considerable quantity of vapour is given off, due 

 to the volatilising of the hygroscopic films. 



The difficulty of the entry of the water into the cells 

 of the outermost layers of the young roots involves the 

 development of a special absorptive 

 mechanism upon them. This takes the 

 form of a number of delicate outgrowths 

 of the internal cells, which form long 

 thin-walled hairs (fig. 54). These are 

 not distributed all over the surface of 

 the young rootlets, but are confined to 

 a particular region not far behind the 

 apex. As the delicate branches of the 

 root grow, the root-hairs gradually perish, 

 more being formed continually at about 

 the same distance from the apex. There 

 is thus a continuous renewal of this collec- 

 tion of hairs, which is maintained as long 

 as the root system extends and continues 

 functional. The interspaces of the soil 

 are penetrated by the young roots, the manner of whose 

 growth involves a very close approximation of their sub- 

 stance to the surface of the particles of which the soil con- 

 sists. The delicate hairs standing out at right angles to 

 the surface of the roots are consequently brought into very 

 close and intimate relations with these particles and with 



FIG. 54. ULTIMATE 

 BRANCHES OF A 

 ROOT, SHOWING PO- 

 SITION OF ROOT- 

 HAIRS. 



