i6 THE PHYSIOLOGY OF PLANTS 



termed the bundle-sheath {endodermis), and when the soft 

 cortical tissues of the roots are destroyed, it keeps the central 

 cylinder intact. 



As a matter of fact, the cortex of the root of many grasses 

 and other Monocotyledons dies off at an early period, and then 

 the root continues to live and function protected by the endo- 

 dermis alone. 



But a matter of great importance in the structure of the 

 root of Allium ascalonicuvi (Fig. 3), and one deserving special 

 mention, is the fact that the interruption of this protective 

 sheath by thin-walled cells (Fig. 3 d) always takes place 

 immediately opposite a group of wood vessels. 



For if we remember that these thin-walled cells offer an 

 easy passage for watery fluids, while the thick-walled cells 

 are rendered impervious by their thick cell-walls, we see that 

 these cells (d) constitute the points of passage of the water 

 which has been taken up by the root-hairs, and, after travers- 

 ing the cortex, has to make its way into the wood vessels, and 

 through them up into the stem and its branches. 



(d.) The Process of Conductioii. 



Having recognised the thin-walled cells of the endodermis 

 as passages through the thick-walled protective sheath of the 

 root, we have gained a sufficient insight into the structure of 

 the root, and can now pass on to the consideration of the 

 physical forces which must necessarily come into play in the 

 passage of the nutritive solution, in the first place into the 

 root, and thence upwards into the leaves. 



But before we begin to examine these processes, let us recall 

 to our mind the conditions of Fig. i. The young root there 

 figured may be looked upon as a compound organ consisting 

 of various parts. First of all we have the actual body of the 

 root, the centre of growth of which lies in the tip near D, P, 

 and Fl. There the cells are in a meristematic condition, like 

 those shown in Fig. 5. 



These cells are still very thin-walled and entirely filled with 

 an albuminous substance resembling the white of egg, which 

 is called protoplasm. Embedded in this mass we find a 



