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. dichotuma and fectinata, 
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. 
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Fic. 314. 
Transverse section of the base of the petiole of Gleichenia dicarpa, showing the 
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. (Platyzoma) microphylla: 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 Géerchenta, 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. 
