46 



STRUCTURAL BOTANY 



showing the steles entering them obliquely from the 

 stem (see Fig. 21). 



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 



FIG. 22. Transverse section of small stele fi;om the petiole of 

 the Male Fern, c, c, cortical cells ; e, endodermis ; st, starch 

 sheath, made up of inner endodermis and pericycle ; ph, 

 phloem ; x, x, xylem ; px, protoxylem ; x.p, xylem-paren- 

 chyma. Magnified 200 diameters. (R.S.) 



of this kind are often called concentric bundles. As, 

 however, in many cases each of them almost exactly 

 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. 



