312 



FILICALES 



[CH. 



be regarded as a stage in a series leading to another well-marked 

 type, the solenostele. The solenostele\ a hollow cyclinder of 

 xylem lined within and without by phloem, pericycle, and en- 

 dodermis, occurs in several genera belonging to different families, 

 e.g. Dipteris, species of Pteris, species of Lindsaya, Polypodium, 

 Jamesonia, Loxsoma, Oleichenia and other genera. In a smaller 

 number of ferns the stele consists of what may be called a 

 meduUated protostele similar to the common form of stele in 

 Lepidodendron: this type is found in species of Schizaea and in 

 Platyzoma (fig. 239). It is important to notice that in the 



Fia. 239. Platyzoma microphylla. l.t., leaf-trace; i.e., internal endodermis. 

 (After Tansley ; modified from Boodle.) 



solenostele and as a rule in the medullated protostele when a 

 leaf-trace passes out from the rhizome stele the vascular cylinder 

 is interrupted by the formation of a foliar gap {Platyzoma^, 

 fig. 239, is an exception). This fact has been emphasized 

 by Jeffrey^ who draws a distinction between the Lycopodiaceous 

 type of stele, which is not broken by the exit of leaf-traces, and 

 the fern stele in which foliar gaps are produced: the former he 

 speaks of as the cladosiphonic type (Lycopsida) and the latter as 

 the phyllosiphonic (Pteropsida). 



The transition to a hollow cylinder of xylem from a protostele 



' Gwjnne-Vaughan (01) ; (03). ^ Boodle (01) p. 73-5. » Jeffrey (00) ; (03). 



