THE TRANSPORT OF WATER IN THE PLANT 69 
the film of hygroscopic water which surrounds them. In 
some cases the pressure between the two is so close that 
the particles become embedded in the membrane (fig. 55). 
The hygroscopic film of water is thus separated from the 
interior of the root-hair by a most delicate pellicle of cell- 
wall substance, lined by an almost equally delicate layer of 
protoplasm. The vacuole of the hair contains a somewhat 
acid cell-sap, by virtue of which osmosis is set up; the 
osmotic equivalent of the acids of the 
sap being considerable, the cell quickly 
becomes turgid and distended, such 
turgescence continuing so long as the 
conditions remain favourable. The root- 
hairs are very numerous, and their united 
action causes a considerable accumula- 
tion of water in the cortex of the root, 
for it passes into the cells of this region 
by osmosis through the base of the hair. 
This, being one of the cells of the ex- 
ternal layer, impinges upon one or more 
of the cortical cells, which have a similar 
reaction to that of the root-hair itself. 
Osmotic currents are thus set up from Fic, 55.—Roor-marr 
every hair, and a gradual accumulation De ea 
of water takes place in the cortex of the 
young root, making all its cells turgescent and causing a 
considerable hydrostatic pressure in the tissue. This tur- 
gescence with its consequent pressure soon extends all along 
the axis of the young root, though it is originally set up only 
by the region which is clothed by the absorbing hairs. 
The central portion of the axis of the root is occupied 
by a cylindrical mass which extends throughout its whole 
length, and which is known as the stele (fig. 56). It is 
generally marked off sharply from the cortex, the cells of 
whose innermost layer, the endodermis, are often peculiarly 
thickened. This thickening is not, however, usually very 
marked in the region of absorption. At certain places 
