68 



VEGETABLE PHYSIOLOGY 



interspaces are filled with air, and a delicate film o 

 surrounds each of its particles and adheres closel 

 This water, often spoken of as hygroscopic water, 

 source of the plant's supply. The presence of ai 

 interspaces supplies the wants of the root and frees 

 the difficulties which have been pointed out. 



The hygroscopic water adheres so closely 

 particles of the soil that it escapes ordinary obs: 

 when, however, soil that has been allowed to d 

 ordinary temperature till its interspaces are 

 empty, is exposed to a heat approaching that 

 water, a considerable quantity of vapour is gi 

 to the volatilising of the hygroscopic films. 



The difficulty of the entry of the water in 

 of the outermost layers of the young roots 

 development of a specia 

 mechanism upon them. 1 

 form of a number of delica 

 of the external cells, whi <r 



thin-walled hairs (fig. 54, 

 not distributed all over W f ^Ux 

 the young rootlets, but 8 

 a particular region not f 

 apex. As the delicate b 

 root grow, the root-hairs g} 

 more being formed contr 

 the same distance from i 

 is thus a continual renew^ 

 tion of hairs, which is mt 

 as the root system exten ' ^es 



functional. The interspaces of the soil 

 are .penetrated by the young roots, the i> aner of whose 

 growth involves a very close approximation of their sub- 

 stance to the surface of the particles of w ^oh 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- 



HAIBS. 



