487 
Origin of the Ulodendroid Scar. 
volume of the cork from which it is made ; that is, that each centimetre of 
secondary cortex of a tree-trunk will, on fossilization, be only represented by 
o*2 cm. of coal film. To take as an example the thickest secondary cortex 
of any petrified Lepidodendron represented in the Manchester collections, 
we find that the secondary cortex of the Dalmeny tree of L, Wunsckianum , 
which is 4*5 cm. through, would be represented by a coal film i cm. in thick- 
ness. Coal films whose thickness is of this order are often found in connexion 
with large lepidodendroid trunks. 
M. Renier’s first specimen is easily explicable on the theory that the 
branch is attached to the whole area of the scar. The description of it as 
showing the quincuncial arrangement of the leaf-trace scars of the trunk 
continued on to the scar is not quite correct. What actually happens is 
that the multispiral arrangement of the leaf-trace sections on the scar, which 
are obviously related to the vascular bundle of the branch, is continued on 
to the ordinary surface of the trunk. This means simply that these latter 
traces arise from the branch stele after it has parted from that of the trunk. 
This explanation applies, whichever theory of the scar be held, and the facts 
are familiar in structural material; cf. Professor Weiss’s figures of the 
‘ biserial Halonia \ 
The suggestion that it necessarily indicates the similar morphological 
nature of the inner surface of the scar-tissue and the outer cortex of the 
trunk is not true, for it is obvious that the intersection of the leaf-traces by 
a thick abscission tissue, such as is postulated on my theory of the scar, 
will produce results closely resembling their intersection by the secondary 
outer cortex. 
Although I am not quite convinced of the accuracy of M. Renier J s 
statement that the leaf-trace sections of the inner and outer surfaces of the 
ulodendroid scars of his second specimen do not correspond, I will accept 
it for the sake of argument. 
I find the thickness of the coal film covering a ulodendroid scar, in the 
comparatively few (about fifteen) specimens still retaining such a film which 
I have been able to examine, to be about i mm. 
I have already shown that this must represent a tissue in the plant at 
least 4 mm., or more probably 5 mm. thick. This would on my theory 
represent the thickness of the abscission tissue. Except rarely in the lower 
part of scars, the leaf-traces will never enter this tissue at right angles ; they 
generally do so at a considerable angle. If they do so at 45 °, then in 
passing through the tissue of the scar, which I have shown above to have 
been in all probability at least 4 mm. in thickness, each will have moved 
outward a distance of 4 mm., which is ample to explain the differences in 
position of the sections on the two surfaces of M. Renier’s scar. 
M. Renier’s third piece of evidence is of lesser importance. I shall 
show later on in this paper that the scars of leaf-traces may be represented 
