THE DIFFERENTIATION OF THE PLANT-BODY 21 
supplemented by masses or sheaths of hardened or scler 
enchymatous parenchyma or even by sclerenchyma itself 
In forms which are intermediate 
in requirements, such as the -@QCXCOS 
petioles of leaves, layers of POY 
collenchyma are developed below = Oe ; 
the epidermis (fig. 25). ‘S 
Sometimes sheaths or layers 
of sclerenchyma are developed 
instead of cork; this condition 
occurs especially among the 
stouter Monocotyledons. 
The protective mechanisms Fic. 25.—CoLLENCHYMA UNDER 
developed by roots also show a = T#=_Epiprrmis or Prrioue 
good deal of variety. The outer- ita 
most layer does not at first take the form of an impervious 
membrane ; this would be inconvenient in view of the neces- 
sity for the existence of root-hairs. In some cases the second 
layer later undergoes modification, its cells becoming thick- 
ened in a peculiar manner ; it then constitutes the exoder- 
mis (fig. 26). Other sheathing layers — 
are also found more deeply seated, 
while eventually the pericycle becomes 
the place of formation of corky tissue. 
The second prominent differentia- 
tion which presents itself is the forma- 5... 96 —snortoy or OUTER 
tion of a system of cells and vessels for Becion or Root, suow- 
InG EXODERMIS, ex. 
the transport of water through the 
plant and the circulation of nutritive and other materials. 
We may speak of this as the conducting system. A little 
reflection will show us the necessity for the development of 
some such system as this, which must be more extensive and 
complex as the size of the plant increases. We find that the 
source of water on which a terrestrial plant relies is the soil 
in which its roots are embedded. Even when it is young 
many of its protoplasts are placed at a considerable dis- 
tance from such a source of supply, and in the absence of 
