XX] RECENT FERNS 315 



an internal endodermis, a feature which leads Jeffrey to what 

 I believe to be an incorrect conclusion that the vascular arrange- 

 ment found in Osmunda regalis has been evolved by reduction 

 from a stele in which the xylem was enclosed within and without 

 by phloem. New facts recently brought to light enable us to 

 derive the ordinary Osmundaceous type from the protostele 

 and solenostele. It is worthy of remark that the Osmundaceae 

 occupy a somewhat isolated position among recent ferns ; their 

 anatomy represents a special type, their sporangia differ in 

 several respects from those of other leptosporangiate ferns and in 

 some features Osmunda and Todea agree with the Eusporangiate 

 ferns. The possession of such distinguishing characters as these 

 suggests antiquity ; and the facts of palaeobotany, as also the 

 present geographical range of the family, confirm the correctness 

 of this deduction. 



Before leaving the stelar structure of leptosporangiate fern 

 stems, a word must be added in regard to a type of structure 

 met with in the Hymenophyllaceae. In this family Trichomanes 

 renifcn-me (fig. 237, E) may be regarded, as Boodle suggests, as 

 the central type: the stele consists of a ring of metaxylem 

 tracheae, the dorsal portion having the form of a flat arch 

 and the ventral half that of a straight band. This flattened ring 

 of xylem encloses parenchymatous tissue containing scattered 

 tracheae some of which are protoxylem elements. In Tricho- 

 manes radicans the rhizome is stouter than in T. reniforme and 

 the stele consists of a greater number of tracheae. The stele 

 is cylindrical like that shown in fig. 238, but the centre is oc- 

 cupied by two groups of protoxylem and associated parenchyma. 

 In ffymenophyllum tunhridgense the stele is of the subcol- 

 lateral type; the ventral plate of the xylem ring has disappeared 

 leaving a single strand of xylem with endarch protoxylem and 

 completely surrounded by phloem. Trichomanes muscoides 

 possesses a still simpler stele consisting of a slender xylem 

 strand with phloem on one side only. Reference has already 

 been made to the occurrence in this family of the protostelic 

 type. The Hymenophyllaceae afford a striking illustration 

 of the modification in different directions of stelar structure 

 connected with differences in habit, and of the correlation of 



