Boodle . — Anatomy of the Gleicheniaceae. 737 
In the pinnae, which probably represent the pinnules of 
species of Gleichenia , the upper epidermis is thick-walled, 
and the mesophyll lacunar. The form &c. of the pinnae 
has been described in the section on the habit. 
The roots are diarch. 
Conclusions. 
Owing to lack of space, general considerations on the 
structure of the Gleicheniaceae must be treated very shortly 
in the present paper. 
Two possibilities must be kept in view: (1) That the 
protostelic condition of most Gleichenias is primitive and that 
the solenostelic type of G. pectinata has been derived from it ; 
or, (2) that the protostelic Gleichenias owe their structure to 
reduction from solenostely. Taking the second assumption, 
the nodal islands of the protostelic Gleichenias might be 
regarded as ancestral remnants of a previous solenostelic 
structure. On the first supposition, on the other hand, the 
nodal islands might be looked upon as local complications of 
the stele, directly connected with the insertion of the leaf- 
trace. The further theory may be added, that the solenostelic 
structure of G. pectinata may have originated through the 
formation of a more bulky nodal island (containing phloem, 
sclerenchyma, &c.) and the continuation of the structure found 
in it through the internode. This is practically similar to the 
explanation given by Poirault (’ 93 , p. 182) of the origin of 
the structure in Platyzoma. The node is often regarded as 
being prone to show ancestral characters. This may be true 
in certain cases, and the reverse in others. The importance 
of the effect of the leaf in moulding the structure of the stem 
has been pointed out by Jeffrey (’00) and by Gwynne- 
Vaughan (’ 01 , p. 87). The requirements to be fulfilled in 
conduction and mechanical support would be somewhat more 
complicated at the node than in the internode, and might well 
lead to greater complexity in structure at the node, so that 
new types of structure would be liable to appear there. If the 
