DIFFERENTIATION OF TISSUES 



285 



in length. The chief mechanical tissue for parts which 

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

 2G5). The cells are thick-walled, and usually elongated 

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



Fig. 2G8. 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; A', xylem, containing spiral (.«, s') aiid pitted (t, V) vessels; 

 C, cambium; P, phloem, containing sieve vessels {sb)\ b, a mass of ])ast fibers or 

 sclerenchyma; ic, pith rays between the bundles; e, tlie bundle sheath; ^, cor- 

 tex.— After Vines. 



Stele. — The characteristic feature of the stele or central 

 cylinder is the development of the mestome or vascular 



