CALIFORNIA COLLEGE 

 of PHARMACY 



THE DIFFEBENTfATION OF THE PLANT-BODY 31 



main ones form an interrupted cylinder (tig. 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 scleren- 



FIG. 30. LEAF OF Pinus (ONE OF THE CONIFEKS) 



ep, epidermis; hy, layer of sclerenchyma ; en, endodermis ; v.b. vascular 

 bundle ; r.d., resin duct. 



chyma 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. 



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 



