26 VEGETABLE PHYSIOLOGY 



this mode of disposition. The two main ones form an 

 interrupted cylinder (fig. 29), so arranged as to protect the 

 delicate vascular tissue, which is in great part placed either 

 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 



e/7 



sc ^L- 



FIG. 29. TRANSVERSE SECTION OF RHIZOME OF THE BHACKEN FERN. 

 x 10. 



sc, bands of sclerenchyma ; hy, hypodermal sheath of sclerenchyma ; 

 st, steles ; ep, epidermis. 



of the stem, being occasionally seen. 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 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 



