346 Bower . — On the Structure of the Axis 
firm layer is not clearly differentiated, though the central 
stele is surrounded by a tissue which is slightly more dense 
than that which occupies the middle of the cortex ; here 
a very peculiar condition of the tissue is found, for it consists 
chiefly of multicellular filaments which are intertwined 
together in irregular fashion (Fig. t 3) ; the tissue resembles 
in its general character that of the central strand of Fitcus 
rather than the tissue of a vascular plant. There is no 
special trabecular development of the tissue round the leaf- 
trace bundles as they traverse the cortex in Lep.fuliginosum . 
From the above it will be gathered that there is some 
variety of detail in the characters of the cortex in the strobilus 
and vegetative axes of Lepidodendron , the differences chiefly 
depending upon (1) the relative thickening of the walls, and 
(2) upon the size of the intercellular spaces, and the position 
in which they appear relatively to the vascular system. There 
is, however, in most cases a differentiation of the cortex into 
three more or less distinct bands, of which the middle band 
was of a softer and often of a very spongy or even trabecular 
character. 
In connexion with the structure of the cortex in various 
species and specimens of Lepidodendron , a comparative 
examination of the cortex in living species of Lycopodium 
and Selaginella is important. It is well known that there are 
considerable differences in the character of the cortex in 
various species of these genera, and it will now be pointed out 
that these are in many respects similar to those found in the 
fossils. In some species of Lycopodium the cortex is not 
clearly differentiated into distinct bands ; this is the case in 
L. annotinum , where it is almost uniformly dense and thick- 
walled ; on the other hand, in L. carinatum , L. Hippuris , 
and L. Phlegmaria the cortex is almost uniform in texture 
throughout, but the tissue is thin-walled, and permeated by 
intercellular spaces of small size. The same is the case in 
Z. dichotomum , at least while young, though in old stems, 
where the roots pursue their internal course downwards 
through the cortex, large mucilaginous cavities are apt to 
