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. 



ej>~ 



FIG. 29. TRANSVERSE SECTION OF KHIZOME OP THE BRACKEN FERN. 



x 10. 



sc, bands of sclerenchyma ; %, 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. 32). In the simplest cases the 

 sclerenchyma is developed in connection with only one of 

 the three regions already alluded to. The stem of Equi- 

 setum and the leaves of Conifers are furnished with a 

 layer of thick-walled cells immediately under the epidermis 



