Boodle . — Anatomy of the Gleicheniaceae . 739 
become small and crowded ; it is perhaps probable that it 
may have been derived from a solenostelic form, by oblitera- 
tion of the leaf-gaps, and disappearance of the internal phloem 
of the stele. This view is, I believe, held by Dr. E. C. Jeffrey. 
It is possible on the other hand that Platyzoma may have 
been derived from a protostelic Gleichenia , and its structure 
might then be due to the formation of a pith and internal 
endodermis. Although the nodal island in Gleichenia is 
here held not to be a remnant of solenostelic structure, some 
forms, e. g. G. circinata , seem to show reduction of the nodal 
island. 
The two small nodal islands of G . pectinata further com- 
plicate the question. Assuming that they do not represent 
the nodal island of other Gleichenias split into two, but that 
the solenostelic structure is due to extension of the original 
nodal island, a similar extension (accompanied by fusion) of 
the small nodal islands of G. pectinata , so as to assume tubular 
form, would produce a double vascular ring like that described 
by Seward (’99) in his interesting paper on Matonia pectinata. 
This plant is perhaps not so very far removed from the 
Gleicheniaceae in its affinities. 
Shortening of internodes and decrease of rate of growth 
of the stem are probably two factors, which both tend to 
abolition of spiral protoxylem elements. This is illustrated 
by Platyzoma and G. moniliformis , where spiral elements are 
absent, and both factors are likely to have been brought 
into play by the strongly xerophytic conditions, of which 
the habit and habitat of the two plants give evidence. The 
shortening of the internodes is not so manifest in G. monili- 
formis, but the change to its upright stem from the 
elongated rhizome of other species would probably greatly 
favour it. 
One or two structural characters in the Order have a bearing 
on stelar theories. The sclerotic tissue with its endodermis 
in the nodal island and in the leaf-bundle is no doubt a 
homologous structure in the different species. Yet it may 
be connected with the cortex in some species, and not in 
3 c 
