Lycopodiales. 15 3 
it seems very likely that the leaf of Sigillaria was characterised by 
two bundles, the question naturally arises whether the single 
median bundle of the leaf of Lepidodendron is primitive or due to 
reduction. So far we have little evidence on this point, though it 
is worth noting that the genus Lepidodendron is, as a whole, more 
ancient than Sigillaria. But whichever genus is the more primitive 
their close affinity is indisputable. One cogent reason for assuming 
a close affinity of the two genera lies in the apparent identity of 
their subterrannean organs, known to botanists as Stigmarias. 
They are dichotomous axes bearing monarch dichotomous appen¬ 
dages. The most important difference between the main Stig- 
marian axes is that in most species the whole of the xylem is 
centrifugal, while, in a few cases, a small amount of centripetal 
xylem occurs internally to the centrifugal wood. In a species 
recently described by Mr. Weiss this centripetal xylem is remark¬ 
ably well developed (27). On grounds of comparative anatomy it 
is clear that the centripetal primary xylem is primitive, and that 
the forms in which the whole of it is replaced by a pith show a 
phylogenetic advance. It is, however, strange that the under¬ 
ground organs of so ancient an order as the Lepidodendraceas 
should commonly have lost all traces of this centripetal xylem ; but 
the Stigmarias appear to have been more highly differentiated than 
the stems to which they were attached. This is evinced by 
Professor Weiss’ demonstration that there are, in the various types 
of appendage borne by the Stigmarian axes, groups of spiral 
tracheides in the cortex connected with the protoxylem by narrow 
bands of spiral tracheides (26). Professor Weiss has shown that 
this is a secondary modification of the mid-cortical region, tending 
to improve conduction, but it serves to show the high degree of 
differentiation which the Stigmarias had reached, and therefore to 
explain the frequent absence of centripetal xylem in them. 
The fructifications of the Lepidodendraceas seem usually to 
have been cones ; but Professor Bower in his latest book announces 
the forthcoming publication by Mr. Kidston of the description of 
a large tree-like type known as Pinakodendron in which the sporo- 
phylls are not disposed in cones, but in which the sporangia are 
are associated with the ordinary leaves of certain portions of the 
stem (4). We do not as yet know if this fossil belongs to the 
Lepidodendraceas; this, however, will he easily settled when Mr. 
Kidston’s description appears, for the Lepidodendraceas are sharply 
characterized by the comparatively simple anatomy of their stems 
