894 PROFESSOR W. C. WILLIAMSON AND DR. 1D. H. SCOTT ON THE 
development of the branch abortion took place. Possibly it never advanced much 
beyond the condition of a bud, more probably it formed a shoot of limited duration, 
comparable to the short leaf-bearing shoots of Pinus. 
This then we believe to be the explanation of the apparent intrusion of tracheides 
into the middle of the branch. The tracheides which appear to penetrate into the 
branch, represent, in our opinion, the commencement of that formation of callus-like 
wood, by which the base of the lateral shoot soon became completely enclosed. 
We have gained, then, this additional fact respecting the branches of Calamutes : 
in many cases they were abortive, or of short duration, and their bases then became 
shut in, like “ knots,” by the wood of the parent stem. 
One specimen, previously figured,* shows a branch somewhat different from those 
already described. The stem and branch are enclosed within a common zone of 
secondary wood, and the sections show that the dimensions of the two are 
approximately equal, as is also the number of their vascular bundles. Evidently we 
have here to do with a different order of branching from that which we have just 
considered. In this specimen the branch repeats the characters of the main stem, and 
was presumably of equal morphological importance ; in the previous cases the branches 
were relatively small lateral appendages, and probably of limited duration. 
As regards the mode of origin of the branches, we are necessarily without any direct 
evidence. Now, however, that the continuity between the primary tissues of the 
branch and those of the main stem has been demonstrated, there can be no doubt that 
the ramification took place while the axis was still young. In fact we have every 
reason to suppose that the branching was normal, not adventitious, and that the lateral 
shoots arose, like the normal branches of Hqwisetum or any other vascular plant, in 
the immediate neighbourhood of the growing point. The secondary wood in which 
the base of the branch is imbedded, was evidently deposited after the branch had been 
formed ; the stem and branch together became invested simultaneously by a common 
woody zone. In fact the conditions in Calamites are the same as in the branching of 
any stem with secondary growth in thickness, and present no special difficulties, now 
that the facts are known. 
The successive changes in structure which we find in a branch, as we trace it from 
its base upwards, have already been described. The changes consist essentially in a 
rapid increase in the diameter of the pith, and in the number of vascular bundles 
surrounding it. 
These facts, which have been already demonstrated by the study of specimens 
showing structure, are exactly reproduced in those medullary casts which represent: 
the pith-cavity of lateral branches. After the fundamental truth had been established, 
that the sandstone and other allied specimens, which were long believed to be the 
actual stems of Calamutes, were merely inorganic casts of a central cavity from which 
* Witwiamson, “ Organization,” Part 1X., Plate 21, fig. 31, A and B (C.N, 102), 
