Stephenson. — Young Stages of Dicksonia and Cyathea. 11 



Protoxylem. 



The spiral elements of the petiole just join on to the stem, 

 hut the elements of the stem are scalariform. 



In fig. 41 the two cells px are the protoxylem of the next 

 leaf ; these cells die out in the section lower down. Fig. 44 

 shows the stem protoxylem, but, as in Loxsoma (Gwynne 

 Vaughan), these elements are scalariform. 



Stelar Structure. 



Up to the last few years consideration of the stele has been 

 on the lines laid down by Van Tiegheai, but lately more attention 

 has been paid to the vascular structure of ferns, and a study 

 of the ontogenetic development has modified the old stand- 

 point. For instance, Jeffrey evidently considers the polystelic 

 structure to be derived from the protostele through the siphono- 

 stele. For in an abstract (Proc. Roy. Soc.) of a paper (full 

 paper not seen) which appeared in the Phil. Trans. Roy. Soc. 

 there is the following : "Starting from the conception that the 

 polystelic structure does not originate by the repeated bifurca- 

 tion of the epicotyledonary central cylinder, but that the latter 

 first becomes a concentric fibro- vascular tube, with gaps for 

 the branches alone . . ." And in a note in the " Annals 

 of Botany " — " Lindsaya, a new type of fern stele " (Tansley 

 and Lulham) : " Thus Lindsaya seems to furnish a phytogenetic 

 link hitherto wanting between the protostele and solenostele, 

 and this view is distinctly supported by the occurrence of the 

 same stage in the ontogenetic series." 



Thus the old views are being modified. The single strands 

 no longer make us overlook the conducting system as a whole. 

 The internal parenchyma is excluded from the stele (Jeffrey) ; 

 the endodermis is no longer regarded as of great morphological 

 importance ; and a study of the ontogeny is held to be necessary 

 for the right understanding of any form (c/. Farmer and Hill 

 — Angiopteris : ; ' It would appear to be probable that no right 

 understanding of a difficult vascular structure is possible apart 

 from a study of its ontogenetic development "). 



The presence, then, of the protostele in the early stages 

 of modern types, and the persistence of the protostele in forms 

 like Gleichenia and Schizcea, point to the protostele as the earliest 

 form of stele. But there are two questions — (1.) Is this proto- 

 stele made up of leaf-traces, or is it partly cauline ? (2.) And 

 how did the transition to the solenostele take place ? 



(1.) In forms with crowded leaves like Cyathea and Dicksonia 

 it would be easy to agree that differentiation of the stele followed 

 the differentiation of the petiole bundles ; and in the earliest 



