THE STEUCTUEE OF THE EOOT 29 



pressure in a lateral direction of the columns of water of 

 adjoining vessels is effected by the valve-like action of the 

 bordered pits, and the slow movement of water through the 

 insterstices of the cell membrane. 



These functions are only performed by the cells and vessels 

 of the wood (xylem) contained in the central cylinder of the 

 root. The bundles, however, possess also a soft bark (phloem), 

 which is concerned in the passage of the organic substances. 

 We shall discuss the method of functioning of this portion 

 of the bundle in dealing with the structure of the stem, and 

 shall now pass on to the- consideration of the, substances which 

 enter the plant dissolved in the water taken up by the roots. 



