560 Bower.—On Medullation in the Pteridophyta. 
But sections about the base of the strobilus show occasional thin-walled 
cells irregularly interspersed among the tracheides (PL XLVII, Fig. 1). The 
protoxylems are still numerous and peripheral and project slightly, while 
the xylems of the leaf-traces on entering the stele fuse directly, though 
often obliquely, each with a protoxylem which descends from a higher 
region of the axis. In sections taken at a higher level the thin-walled 
elements increase in number, forming a considerable though irregular tract 
towards the centre of the stele. This is, however, continuous outwards as 
irregular, thin-walled rays; these connect with the outer conjunctive tissue 
(Fig. 2). Occasionally an isolated tracheide may be found embedded in 
the soft central tissue (Fig. 4), or more than one of them (Fig. 6). Higher 
up (but still in a region where the tissues are already matured and the 
sporangia are in an advanced state, bearing megaspores with thick, dark- 
coloured walls) the central thinner-walled tissue predominates, and the 
xylem is divided into distinct strands, with broad thin-walled rays inter¬ 
vening between them. Figs. 3 and 6 show this. In P'ig. 4 it is seen that 
the insertion of a leaf-trace impinges slightly obliquely upon one of the 
strands. Longitudinal sections show that the softer elements are elongated, 
with protoplasm and nucleus. Occasional transverse walls occur, but there 
is no conspicuous shortening of the cells, as is the case in the pith of most 
plants. This tissue may accordingly be held to represent degraded xylem. 
It is thus seen in a living plant that a change of structure appears, 
involving the formation of a soft, pith-like intrastelar tissue, and it is to be 
noted that it occurs in a case where there is no branching of the axis, nor 
are there any foliar gaps to provide that continuity with the cortex without 
which cortical intrusion cannot take place. Moreover, the trabecular zone 
intervenes between the stele and the cortex uninterruptedly. It may of 
course be asserted that the softer central tissue is not really a pith because 
the cells are elongated. They are, however, thin-walled, and retain their 
protoplasm, and form a central living tract of tissue which is not vascular. 
These are the essential features of a pith. It may also be urged that the 
soft tissues are merely the result of imperfect development of tracheides, 
and that they would mature into tracheides later. Against this it is to be 
remarked that the tissues have the appearance of maturity, while the con¬ 
dition of the sporangia and of the strobilus as a whole shows that further 
development is not to be expected. This intrastelar tissue in X. spimdosa 
may in fact be held as comparable with the pith seen in certain Lepido- 
dendreae. And a very remarkable parallel is seen between the slender 
cone of this Selaginella and the massive cone of Lepidostrobus Brownii , in 
respect of the internal structure. 1 I have shown elsewhere 2 how in this 
1 It may be noted also that a pith exists in the stele of the cone of Spencerites (Miss Berridge, 
Ann. Bot., xix, p. 274, Fig. 2). Doubtless other examples may be found in the cones of fossil Lyco- 
podiales. 2 Ann. Bot., vii, p. 347, PI. XVI, XVII. 
