THE TEANSPORT 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 as well as root, reaching 

 indeed 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- 

 position 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 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 mass of some size. 



FlG. 57. DlAGKAM SHOWING 



COURSE OF THE VASCULAR 

 BUNDLES IN A DICOTYLE- 

 DONOUS PLANT. 



