XX] RECENT FERNS 311 



to a type of stele consisting of a central core of xylem surrounded 

 by phloem, pericycle, and endodermis. While admitting that 

 steles of this type may sometimes be the result of the modifica- 

 tion of less simple forms, we may confidently regard the 

 protostele as representing the most primitive form of vascular 

 system. The genus Lygodium affords an example of a proto- 

 stelic fern; a solid column of xylem tracheae and parenchyma is 

 completely encircled by a cylinder of phloem succeeded by a 

 multi-layered pericycle and an endodermis of a single layer of 

 cells. In this genus the stele is characterised by marginal 

 groups of protoxylem ; it is exarch. An almost identical type is 



Fig. 238. Stele of Trichomanes scandens : px, protoxylem ; s, endodermis. 

 From Tansley, after Boodle. 



represented by species of Gleichenia, but here the stele is 

 mesarch, the protoxylem being slightly internal (fig. 237, C). 

 Trichomanes scandens (fig. 238) has an exarch protostele like 

 that of Lygodium; but, as Boodle' has suggested, the protostelic 

 form in this case is probably the result of modification of a 

 collateral form of stele such as occurs in Trichomanes reniforme 

 (fig. 237, E). A second type of stele has been described in 

 species oi Lindsaya'' in which the xylem includes a small group 

 of phloem near the dorsal surface. This Lindsay a type is often 

 passed through in the development of " seedling " ferns and may 

 1 Boodle (00). - Tansley and Lulham (02). 



