46 



STRUCTURAL BOTANY 



Every stele consists essentially of a central mass of 

 wood surrounded by a ring of phloem (see Fig. 22). 

 This arrangement is usual in Ferns, and vascular strands 



O ' 



of this kind are often called concentric bundles. As, 

 however, in many cases each of them almost exactly 



FIG 22. Transverse section of small stele from the petiole of the Male 

 ern. c, c. cortical cells ; e, endoderniis ; s/, starch sheath, made up 

 of inner endodermis and pericycle ;ph, phloem ; x, x, xylem ;i>.x, pro- 

 toxylem ; x.p, xy tern-parenchyma. Magnified 200 diameters. (R.S.) 



repeats the structure of the whole vascular cylinder of the 

 embryonic stem, it is more convenient to describe them as 

 steles or cylinders, but no sharp distinction can be drawn. 

 The wood consists of vessels and parenchyma ; the 

 vessels of Ferns are generally of the kind called sea&m/brm, 

 or ladder-like, from the peculiar structure of their walls, 

 shown in Fig. 23. This structure depends on the form of 



