292 STRUCTURE OF FERN-STEM 



be, bearing numerous forked lateral veins which are not con- 

 nected by cross-branches (Fig. 163, A, B). 



A general idea of the more characteristic features in the 

 internal structure of the stem can be gathered from a study of 

 the Bracken rhizome. In transverse section (Fig. 158, A) this 

 is seen to be traversed by two rings of vascular strands or 

 steles (St.). Separating the rings are two bands of dark brown 

 sclerenchyma (Scl.), and the same tissue also forms a layer 

 beneath the epidermis ; its elements (Fig. 161, Scl.) differ from 



EJ.S. 



p*> 



FIG. 1 60. Part of a stele from the rhizome of the Bracken (Pleris aquilina) 

 in transverse section. En., endodermis ; P., pericycle ; P.p., phloem- 

 parenchyma ; P.xy., protoxylem ; S., sieve-tube ; X.p., xylem- 

 parenchyma ; Xy., xylem. 



the fibres of higher plants in being short and relatively thin- 

 walled. The remaining ground tissue is parenchymatous, and 

 contains an abundance of starch. 



The vascular strands are concentric (Fig. 160), with the 

 component tissues more or less symmetrically arranged. The 

 term stele is customarily applied to these concentric strands of 

 Ferns, but this does not necessarily imply that they are com- 

 parable to the entire vascular cylinder of a higher plant. Each 

 stele is bounded by an endodermis (En.), with dark brown mem- 

 branes, and a pericycle (P.), composed of rather larger thin- 



