DIFFERENTIATION OF TISSUES 



285 



in length. The chief mechanical tissue for parts which 

 have stopped growing in length is sclerenchyma (Figs. 264, 

 265). The cells are thick-walled, and usually elongated 

 and with tapering ends, including the so-called "fibers." 



FIG. 268. Sections through an open collateral vascular bundle from a sunflower stem; 

 A, cross- section; B. longitudinal section; the letters in both referring to the same 

 structures; M, pith; JT, xylem, containing spiral (s, s') and pitted (t, t') vessels; 

 C, cambium; P, phloem, containing sieve vessels ($b); b, a mass of bast fibers or 

 sclerenchyma; ic, pith rays between the bundles; e, the bundle sheath: It, cor- 

 tex. After VINES. 



Stele. The characteristic feature of the stele or central 

 cylinder is the development of the mestome or vascular 



