Notes. 
163 
indications of the steps by which protostelic structure was changed 
into the Lindsaya-Xy pe. Mr. Boodle has described in his last 
paper 1 the structure of the stele of the node of Gleichenia (sub- 
genus Mertensia) flabellata. As the node is approached from 
below, a ‘ nodal island ’ of parenchyma, in the midst of which are 
some sieve-tubes, appears in the xylem of the stele near its dorsal 
surface, so that at this level the structure of the stele in the essential 
relations of xylem and phloem is identical with that of Lindsaya. 
The description of the arrangement of tissues above this point, 
involving the detachment of the leaf-trace, strikingly agrees with 
the facts we have described in the case of Lindsaya , the main 
differences being the much bulkier and sclerotic strand of tissue 
enclosed by the internal endodermis in G . flabellata , and the fact 
that the two endodermes oin in the latter only after the xylem and 
phloem of the meristele have become detached from the stele at the 
other end ; in this last respect G. ( Eugleichenia ) dicarpa is much more 
like Lindsaya (see Boodle, Fig. 22). 
The petiolar meristele of Gleichenia is, we are inclined to believe, 
primitively collateral in structure, i. e. with phloem belonging only to 
its outer (lower) side. The internal phloem, which passes round the 
incurved ends of the meristelic xylem, as seen in cross-section, and 
in extreme cases (G. dicarpa) becomes almost continuous round the 
concave side of the dorsal bay of xylem, may be regarded as an 
inward extension of the external phloem, developed to meet the 
increasing needs of conduction. But such an extension had to be 
provided with an attachment- to the phloem of the stele. At first its 
attachment to the ordinary external phloem of the stele seems to have 
been mediated by the downward extension of the bay of the meristele 
(G. dicarpa , Boodle, Fig. 20), which in its lower part contains phloem 
alone (cf. our Fig. 1 1). In other cases we must suppose the downward 
extension of this bay became completely enclosed in the xylem of the 
stele, forming a ‘ nodal island ’ consisting not only of phloem but also 
of a downward extension of the central parenchyma or sclerenchyma 
of the meristele, enclosed by an internal endodermis. Of what precise 
benefit to the plant this blind downward extension of the internal 
phloem (or its corresponding upward extension, described by 
Mr. Boodle in G. dichotoma ) may be, it is difficult in the present 
state of physiological anatomy to guess. But if we imagine the 
1 Boodle, Gleicheniaceae, p. 723. 
M 2 
