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, 
&e. 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—+.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 ; 
