DIFFERENTIATION OF TISSUES 



285 



in length. The chief mechanical tissue for parts which 

 have stopped growing in length is sclerenehyma (Figs. 26-4, 

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

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



Fig. 288. 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: X. xylem. containing spiral (#. #') and pitted U. C\ vessel*: 

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

 sclerenehyma: ie, pith rays between the bundles; e, the bundle sheath; R, cor- 

 tex.— After Vines. 



Stele. — The characteristic feature of the stele or central 



cylinder is the development of the mestome or vascular 

 37 



