490 
Watson. — On the Structure and 
The extra-umbilical part of the scar shows excellently the transition 
from the short, almost circular, sections of the leaf-traces in the lower part, 
to the elongated scars of the upper part of the ulodendroid disc. 
In the lower part, each leaf-trace section is raised on a little cushion, 
which presents a quite illusory resemblance to a true leaf-base. Examina- 
tion of the figure will confirm this. 
In the upper part each leaf-trace is seen to cut the surface of the 
scar as an elongated groove, which dips down into the stem towards 
the umbilicus. Between the point where the leaf-trace has entered the scar 
surface and the centre, the coal film forms a low ridge, which can only be 
due to the persistence of the leaf-trace, either within the scar tissue or below 
it, from the intersection to the umbilicus. This scar therefore affords 
absolute proof that the outer surface of a ulodendroid scar has received its 
leaf-trace bundles from within the trunk. 
The fact shown in this scar, and still more clearly in the other two 
perfect scars of the same specimen, that the leaf-trace sections appear as 
depressions on the upper, and as raised papillae on the lower, part of the 
same scar, shows that the third line of evidence does not hold in the 
present example. 
The other Ulodendron which I wish to describe is, like the first, 
a flattened trunk 20 cm. in maximum width, and 66 cm. long, in which 
distance it bears nine ulodendroid scars on one side, and ten on the 
other. The coal film has been largely removed from this specimen, leaving 
a cast of a sub-cortical surface. On this the areas of the scars are faintly 
marked off in a way not readily intelligible in the light of M. Renier’s 
explanation. 
Three ulodendroid scars show a good deal of the coal film. The outer 
surface of this is well preserved, and confirms the facts observed in the 
other specimen that I have described. They also show that the leaf-traces 
seen in section on the outer surface of a ulodendroid scar have sprung 
from the branch stele whilst still inside the main trunk. 
I believe that these two specimens, together with that which 1 originally 
described, which is not explained by M. Renier, and seems to be inexplicable 
on his theory, are sufficient to demonstrate that the branch was attached to 
the whole area of the scar. 
The structure of the scar as seen in the specimens described above, 
which are probably better preserved than any which have previously been 
figured, its definiteness, and the smoothness of its surface, can only be 
explained, if the branch were attached to the whole area of the scar, by 
calling in the aid of a definite abscission layer comparable to that of leaf- 
fall. That we are justified in doing so is shown by the description below of 
petrified specimens showing such a mechanism. Although it is not certain 
that these actual examples were Ulodendra, the occurrence of cladopsis in 
