THE SEEDLING AND YOUNG PLANT. 29 



cells all alike ; in a few days some of these cells will 

 have changed into constituents of the axis-cylinder and 

 cortex, and subsequently some of them will give rise to 

 vascular bundles, etc. Not all, however ; and it is neces- 

 sary to understand that as the embryonic tissue moves 

 onward 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 discolored, 

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

 stance impervious to moisture, or nearly so ; conse- 

 quently 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 demarkated off 

 from the cortex by a single layer of cells shaped like 

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

 this is called the endodermis, and may be regarded as a 



