26 VEGETABLE PHYSIOLOGY 
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 sclerenchyma can be noticed, 
strands in the middle of the cortical tissue, or in the pith 
of the stem, being occasionally seen. Stems which are 
‘angular in section are usually found to have their angles 
strengthened in a similar way. 
Fic. 29.—TRANSVERSE SECTION oF RHIZOME OF THE BrscKEN FErRn. 
x 10. 
sc, bands of sclerenchyma; hy, hypodermal sheath of sclerenchyma ; 
st, steles; ep, epidermis. 
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 arrangement of 
strengthening material may serve to illustrate this -par- 
ticular differentiation (fig. 82). In the simplest cases the 
sclerenchyma is developed in connection with only one of 
the three regions already alluded to. The stem of Eiqui- 
setum and the leaves of Conifers are furnished with a 
layer of thick-walled cells immediately under the epidermis 
