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-hairs and 

 the turgid cortex, in fact, exert in this way a kind of con- 

 tinuous pumping action, forcing 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. 

 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 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 



Fio. 67. DIAGRAM 

 COURSE OF THE 

 BUNDLES IN A 

 DONOUS PLANT. 



SHOWING 



VASCULAR 



DlCOTYLE- 



