GLEICHENIACEAE 



563 



developed leaf : the formation of the more bulky pockets, and their 

 continuation throughout the internode would give the solenostelic structure. 

 Lastly, the most complex stelar state is seen in G. dichotuina and pcctinafa, 

 species which in the character of the leaf, as well as of the sorus and 

 sporangium, are aberrant from the rest of the genus, and have been 

 recognised as showing advance towards the Cyatheaceous type. These 

 several grounds indicate that an evolutionary progression rather than a 

 retrogression is illustrated in the genus, from a protostelic to a solenostelic 

 structure. 



Transverse section of the base of the petiole of Glcitltenia dicarfm, showing lh<- 

 pseudo-stelar structure resulting from contraction of the horse-shoe-like xylem, till its 

 margins fuse. Photograph by R. Kidston, from section by Gwynne-Vaughan. 



But lastly, there is the case of G. (Platyzonui) microphylln : Boodle 

 suggests that this is a xerophytically reduced form, in which the leaf- 

 traces have become small and crowded, and that it is probably derived 

 from a solenostelic form by obliteration of the leaf-gaps and disappearance 

 of the internal phloem. But his alternative suggestion, that it may have 

 been derived from a protostelic Gleichenia, and its structure be due to 

 the new formation of a pith and internal endodermis, appears the simpler 

 as well as the more probable, in the case of an upright plant with closely 

 crowded leaves. For it must be remembered that this was the condition 

 of the shoot common for primitive Pteridophytes. 



