738 Boodle . — Anatomy of the Gleickeniaceae. 
new structure were suitable for the internode, it might spread 
through that region, by what might be called transformation 
of the tissues there. On the other hand, if reduction should 
take place in the structure of the internode owing to change 
in habit or other causes, a vestige of the previous structure 
might be retained at the node, as being more suitable to the 
slightly different requirements of the latter. The nodal island 
in Gleichenia may therefore be looked upon as either a 
remnant of previous solenostelic structure, or as a local 
modification in the stele, which finally led to solenostely in 
G. pectinata . The latter view on the whole appears the more 
probable. It is favoured by the two facts that the seedling- 
stem up to the third leaf shows protostelic structure, and that 
the leaf of G. pectinata does not carry dichotomy so far in the 
ramification of its frond as the other species, dichotomy in the 
leaf being taken as a primitive character. Much weight must 
not be laid on either of these features, but they are of some 
value, as nothing important has been noticed pointing in the 
other direction. In this connexion it is very interesting to 
note that the sorus of G. pectinata has unusual characters, 
which it shares with G . dichotoma , and in which these two 
species differ from the others of the genus, as described by 
Bower (’99, p. 33 ), namely, that it consists of more numerous 
sporangia. The latter are multiseriate in the sorus, and in 
this respect approach other Leptosporangiate Ferns (Bower, 
’99, p. 34 ). Though, considering the direction of specializa- 
tion in the Leptosporangiate Ferns, the change to multiseriate 
sori may be taken as an advance, the uniseriate sorus might 
in certain cases be due to reduction, so the evidence of the 
sorus does not materially help in solving the present question. 
It will be seen from what has been said above that the 
following suggestions must be put forward in a quite tentative 
manner. The protostelic structure of Gleichenia may be 
regarded as primitive, and the solenostelic type of G. pectinata 
as derived from it, the nodal islands of the protostelic forms 
representing an intermediate step. Platyzoma appears to be 
a xerophytically reduced form, in which the leaf-traces have 
