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 
Fig. 54.— Unrimare jg thus a continuous renewal of this collec- 
BRANCHES OF A 
Root, suowrne Po- tion of hairs, which is maintained as long 
SITION oF Root- . 
HAIRS. 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 
