THE TRANSPOKT OF WATER IN THE PLANT 73 



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 farthest from 

 the tip gradually perish, more being 

 formed continually at about the same 

 distance from the apex. There is thus 



x FIG. 54 ULTIMATE 



a continuous renewal of this collection BRANCHES OF A 



of hairs, which is maintained as long RoOT ' 



POSITION OF ROOT- 



as the root system extends and continues HAIRS. 

 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 



