THE DIFFERENTIATION OF THE PLANT-BODY 31 
main ones form an interrupted cylinder (fig. 29), so arranged 
as to protect the delicate vascular tissue, which ig in great 
part placed cither within this cylinder or in some similar 
relation to other similar sclerenchymatous strands. In the 
case of a plant of humbler type, the common hair-moss 
(Polytrichum), a development of somewhat sclerotised cells, 
forms a central core passing down the stem. In many of 
the flowering plants more complex distribution of scleren- 
Fic. 30.—Lear or Pinus (ONE oF THE ConIFERS) 
ep, epidermis; hy, layer of sclerenchyma; en, endodermis; vb. vascular 
bundle; 7.d., resin duct, 
chyma can be noticed, strands in the middle of the cortical 
tissue, or in the pith of the stem, being occasionally geen. 
Stems which are angular in section are usually found to have 
their angles strengthened in a similar way. 
The arrangement of this sclerenchyma is generally such 
as to supplement the bundles, and to secure the greatest 
amount of solidity and sufficient flexibility, with the least 
expenditure of material. 
Instances of various methods of arrangements of 
strengthening materials may serve to illustrate this par- 
ticular differentiation (fig. 32). In the simplest cases the 
