THE DIFFERENTIATION OF THE PLANT-BODY 25 
latter is worn away. Certain cells become specialised and 
form layers of cork (fig. 23), which arise successively at 
gradually increasing distances from the exterior, and in the 
case of trees finally lead to the construction of a bark (fig. 24). 
The corky formations are supple- 
mented by masses or sheaths of 
hardened or sclerenchymatous 
parenchyma, or even by scleren- 
chyma itself. In forms which 
are intermediate in requirements, 
such as the petioles of leaves, 
layers of collenchyma are de- 
veloped below the epidermis 
(fig. 25). 
Sometimes sheaths or layers He. 26.—CoLLENcHyMA UNDER 
tue EpmErMis oF PETIOLE. 
of sclerenchyma are developed ‘BO. 
instead of cork; this condition 
occurs especially among the stouter Monocotyledons. 
The protective mechanisms developed by roots also show 
a good deal of variety. The outermost layer does not at first 
take the form of an impervious membrane; this would be 
inconvenient in view of the necessity for the existence of 
root-hairs. In some cases the second 
layer undergoes modification, its cells 
fit closely together, and the radial 
walls become cuticularised where they 
are in contact with each other (fig. 
26, ex); it then constitutes the Fic.26.—Sxcrion or OurrR 
exodermis. Later the corky change ee ek 
extends to all the cell-walls of this 
layer. Other sheathing layers are also found more deeply 
seated, while eventually the pericycle becomes the place of 
formation of corky tissue. 
The second prominent differentiation which presents itself 
is the formation of a system of cells and vessels for the trans- 
port of water through the plant and the circulation of nutri- 
tive and other materials. We may speak of this as the 
