Origin of the Ulodendroid Scar . 485 
We are now in a position to examine into the possibility of M. Renier’s 
explanation of the ulodendroid scar. 
If his view be correct, it should be possible to find a whole series of 
specimens of Ulodendron in the youngest of which the umbilicus was of 
nearly the same diameter as the scar, whilst progressively older ones show 
a gradual increase in the diameter of the scar as compared with the 
umbilicus. In the whole series of Ulodendrons which I have examined, 
about two hundred in number, none has an umbilicus more than one quarter 
of the scar’s diameter. This fact alone tells strongly against M. Renier’s 
view. 
In the great majority of Ulodendrons, such, for example, as the 
originals of the figures of this paper, the diameter of the umbilicus is 
about one-fifth of that of the scar. In Bothrodendron this ratio often drops 
to one-eighth. On M. Renier’s theory, the diameter of the umbilicus repre- 
sents the original diameter of the branch, which has increased by secondary 
growth to the diameter of the scar ; that is, the branch must have increased 
to five or more times its original diameter, but we have seen above that the 
actual greatest known increase in diameter of lepidodendroid stem observed, 
in a series which is now very large, is of the order of one-half its original 
diameter ; that is, only one-tenth of the amount required by M. Renier’s 
theory of the ulodendroid scar. 
If we suppose, for the sake of argument, that M. Renier’s theory is a true 
one, and that his ‘ Schema 2 ’ or my Text-fig. 1 represents the condition 
of affairs at the bottom of a ulodendroid branch, then it appears that the 
whole of the stele, inner, and middle cortex of a branch 4 cm. in diameter 
are only in connexion with the corresponding tissues of the trunk by a neck 
5 mm. in diameter, i. e. only one-eighth of the diameter of the branch. 
In my last paper on the ulodendroid scar I showed that the diameter 
of the wood alone, in stems comparable to this in size, is of the order of one- 
eighth of the diameter of the branch ; in other words, the actual wood 
strand of the branch of M. Renier’s ‘ Schema 2 ’ would only just pass through 
the hole into the trunk without contraction, and the rest of the stele and the 
very important middle cortex would be completely cut off. 1 If, on the 
other hand, we imagine the stele of the branch to have retained its putative 
original diameter, then we must suppose that in the mature condition the 
area of the living and conducting tissues of the base of the branch were 
suddenly reduced to #4, i. e. (f), of their section further out, which is mani- 
festly improbable. 
The foregoing discussion will, I think, have shown the improbability 
of M. Renier’s theory from a purely general standpoint. I now propose to 
discuss very briefly the case of calamitian branches, which he adduces as 
analogous to his theory. 
1 Renier, op. cit., p. 49. 
