THE MOVEMENT OF WATER IN PLANTS. 



93 



and in order to do this, we will first of all briefly consider the 

 structure of the root. 



The structure of the root is tolerably uniform throughout 

 the vascular plants. It consists, generally speaking, of a 



FIG. 20. Transverse section of the root of Lepidium sativuin (the Cress) : a, the 

 unicellular root-hairs ; b, the epidermis ; <:, the cortical parenchyma ; d, the 

 axial fibrovascular cylinder ; , the bundle-sheath (endodermis) ; f, the central 

 wood-vessels. 



central fibrovascular cylinder which may or may not enclose 

 a certain amount of pith : this cylinder is surrounded by 

 several layers of parenchymatous cells, and the most external 

 of these layers is in contact with the epidermis, of which cer- 

 tain cells are modified into root-hairs (Fig. 20). Water and 

 substances in solution are absorbed, as we have already seen, 

 osmotically by the root-hairs, and they are transferred in this 



