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 

 sclercnchyma {Set.), and the same tissue also forms a layer 

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



Fig. 160. — Part of a stele from the rhizome of tlie Bracken (Plcris aquilina) 

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

 parencliyma ; P.xy., proto.xylem ; .S"., sieve-tube ; X.p., xylem- 

 parencliyma ; A3'., xylem. 



the fibres of higher plants in being short and relati\-cly thin- 

 wallcd. 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 S3'mmetricall_y arranged. The 

 term stele is customarily applied to these concentric strands of 

 Ferns, but this does not neccssarilj? imply that the}' 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- 



