THE SEEDLING AND YOUNG PLANT 29 



growing-point consists of embryonic cells all alike ; in 

 a few days some of these cells will have changed into 

 constituents of the axis-cylinder and cortex, and sub- 

 sequently some of them will give rise to vascular bundles, 

 &c. Not all, however, and it is necessary to understand 

 that as the embryonic tissue moves onwards and leaves 

 the structures referred to in its wake, it does so by 

 producing new embryonic cells in front i.e. between the 

 present ones and the root-cap. 



We must now look a little more closely into the 

 structure of the axial cylinder, at a level a little behind 

 the region where the root-hairs are produced on the 

 piliferous layer. 



A thin transverse section in this region shows that 

 the root-hairs have all died away, and the walls of the 

 cells of the piliferous layer are becoming discoloured, 

 being, in fact, converted into a brown, cork-like substance 

 impervious to moisture, or nearly so ; consequently the 

 piliferous layer is no longer absorptive, and it will soon 

 be thrown off, as we shall see. 



The cortex offers little to notice, except that its cells 

 are being passively stretched or compressed by the 

 growth and processes going on in the axial cylinder, and 

 it is this cylinder that attracts our special attention, and 

 several points not noticed before must now be examined 

 in some detail. 



In the first place, the cylinder is demarcated off 

 from the cortex by a single layer of cells shaped like 

 bricks, and with a sort of black dot on the radial walls ; 



