Kershaw.—A Fossil Solenostelic Fern . 
687 
is inserted upon the base of the petiole and attached to one margin of its 
vascular strand. An attempt has been made in Fig. 6 to represent a node 
of the rhizome with leaf-gap, and to show the relations between it and the 
leaf-trace and adjacent lateral branch. The restoration is in part hypo¬ 
thetical, for although its position is obvious the actual leaf-trace is not seen 
in any section, and therefore its shape, direction, &c., are merely suggested, 
and possibly not the actual ones. A comparison of Fig. 6 with the series 
represented in the Text-fig. will show the general arrangement of the various 
parts. At a, Text-fig., which is a section below the node, the solenostele 
is seen to be of uniform thickness ; b represents a section a little higher and 
nearer to the node. Fig. 1, PI. LVIII, is a photograph of this same section. 
The solenostele is no longer of uniform thickness around its circumference. 
The dorsal portion which will very soon open to form the leaf-gap is 
slightly thicker than the remaining part of the stele. Diagram c, Text- 
fig., shows this dorsal thickening more markedly. In the next section, 
represented at d , the solenostele has become open in preparation for the 
departure of the leaf-trace, and the free margin of the leaf-gap is seen to be 
considerably thickened, owing to an increase in the number of tracheides at 
this point. The xylem ring at this point is at least twice as thick as in any 
other part, so that the free margin of the leaf-gap projects considerably 
towards the centre of the stem. 
This thickening of the margin is of interest, a similar development 
having been described in several species of Dicksonia and Davallia by 
Gwynne-Vaughan. 1 It is regarded by him as a preliminary step in the 
formation of internal steles. A further step in this direction is seen in 
Dipteris conjugata , described by Seward and Dale. 2 In this fern the thicken¬ 
ing of the margin of the leaf-gap extends through the internodes as well as 
the nodes, and this portion of the xylem has become almost completely 
separated off from the solenostele. 
The anatomy of the fossil shows that Solenostelopteris must have 
occupied a similar position as regards the development of internal steles to 
Dicksonia apiifolia among recent ferns. In both these ferns the develop¬ 
ment is limited to a nodal thickening of the xylem of the free margin of the 
leaf-gap, without any indication of a separation of the thickened part of the 
xylem from the solenostele, as is seen in Dicksonia adiantoidcs and more 
clearly in Dipteris conjugata. 
Section e shows a further change in the shape of the stele preparatory 
to the formation of the lateral shoot given off in connexion with the leaf- 
trace. The stele is seen to be considerably stretched in the horizontal 
plane, the region marked b being destined to form the branch. The limits 
1 Gwynne-Vaughan (’ 03 ), 1 . c. 
2 Structure and Affinities of Dipteris , &c. Phil. Trans., Series B, vol. cxciv, p. 499, and Fig. 4, 
1901. 
