76 VEGETABLE PHYSIOLOGY 
and in the early part of the year, before the leaves of the 
plant expand, they thus become filled with liquid. This 
filtration into the vessels tends to relieve the pressure in the 
cortex, and additional 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-hairg and 
the turgid cortex, in fact, exert in this way a kind of con- 
tinuous pumping action, forcmg it along the axis. The 
force, which is the expression 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 quan- 
tity 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 as well as root, reaching, 
indeed, every cell into which 
osmotic transport can take place. 
Fra. 67.—Diaonam snowrsa = The action of the root-hairs is 
Loken ae lacy thus responsible not. only for the 
DONOUS PLANT. 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 disposition of the woody elements 
is somewhat different in the two regions, there is no doubt 
that they also are continuous throughout (fig. 57). The 
stream of water consequently passes up the woody tissue of 
