THE TRANSPORT OF WATER IN THE PLANT 71 
water can then be absorbed from the soil as before. The 
consequent increase of the turgescence is followed by further 
filtration into the vessels, and these two factors continually 
acting together, the water is made to rise gradually in the 
axial stele. The root-hairs and the turgid cortex, in fact, 
exert in this way a kind of continuous pumping action, 
forcing it along the axis. The force, which is the expres- 
sion of the elastic recoil of the cell-walls of the over- 
distended cortical cells, and which is brought to bear upon 
their fluid contents, squeezing a quantity of liquid through 
the cell-walls into the vessels, is known as root-pressure, 
and is one of the main factors in the transport of water 
through the plant. 
The turgescence not only leads to the rise of the 
sap in the axial stele, but it 
spreads throughout the whole of 
the cortical tissue of the plant, 
stem ag-well as root, reaching 
indeéd every cell into which 
osmotic diffusion can take place. 
The action of the root-hairs is 
thus responsible not only for the 
rapid ascent of the sap, but also 
for the maintenance of turgidity 
outside the region supplied by 
the ascending stream. 
The stele of the root is 
directly continuous with that of 
the stem, and though the dis- 
oe Fie, 57.—D1acRaM SHOWING 
position of the woody elements CQaunée OS ae “Vaecutak 
is somewhat different in the two  BuNruns, IN 4 Diconyne- 
regions, there is no doubt that 
they also are continuous throughout (fig. 57). The stream 
of water consequently passes up the woody tissue of the 
stem so long as the cells are living. The stream in 
young plants passes along the whole substance of the wood, 
which in most cases forms a central mags of some size. 
