1 86 AERATING SYSTEM [CH. 



symmetry of pattern to the transverse section. The process of 

 development of the intercellular spaces has been followed by the 

 present writer in the case of Stratiotes aloides^ (Fig. 121). The 

 whole inner region of the cortex in the root of this plant must 

 be visualised as consisting of radially arranged plates, one cell 

 wide, which in the early stages are so placed as to leave no spaces 

 between. The cells composing the plates divide very rapidly, 

 and a number of new cell-walls are formed, almost all in planes 



, 



-Lac. 



Lacr 



FIG. 121. Stratiotes aloides, L. Tangential section through middle cortex of a young 

 root to show the origin of the lacunae (lac.), (x 318.) [Arber, A. (1914).] 



at right angles to the long axis of the root. The result is that the 

 plates elongate in the direction of growth of the root, but, owing 

 to the rapidity of their cell-divisions, the plates grow in length 

 faster than the rest of the root, and are thus forced into un- 

 dulations, since they become too long to retain their normal 

 vertical position. The possibility of their taking up this sinuous 

 form is due to the fact that the root enlarges in diameter and 

 thus allows room for the separation of the plates. It will readily 

 1 Arber, A. (1914). 



