208 
PROFESSOR W. C. WILLIAMSON ON THE ORGANISATION 
formed alike bj' the one or the other of the two processes, which he respectively 
names schizogenetic and lysigenetic. So far as I can see at present, we must choose 
between these two processes in seeking an explanation of the development of the 
medullary area and the coincident annular expansion of the vasculo-medullary 
cylinder, so characteristic of all the Carboniferous Lepidodendroid plants. 
In the preceding Memoir I have once more shown (figs. 1-6) that a stem or branch, 
the transverse section of which reveals a large and conspicuous parenchymatous 
medulla surrounded by a tracheidal, vasculo-medullary cylinder, frequently can, and 
possibly always does, give off to a much younger and smaller twig a solid axial 
bundle, in the interior of which no traces of a medulla can be seen. I have also 
again demonstrated (figs. 7-15) that a very young twig, destined in the future to 
enlarge into a branch, but in which the axial bundle of tracheids or vessels (fig. 8) is 
solid, not a hollow cylinder, and in which no traces of parenchyma can be detected, 
undergoes changes as it grows older and larger; the axial vascular bundle becomes 
more and more hollow, whilst, in its expanding interior, a cellular medulla, at 
first very small, and consisting of but a few cells (fig. 9), becomes gradually larger 
(figs. 10, 11, 12), and its component cells more numerous, as explained in the Memoir. 
These are indisputable facts, whatever may be the explanation of them. In seeking 
such an explanation, I repeat, we are shut up to the two processes described by 
DE Bary, to account for the expansion of the solid axial vascular bundle into a 
hollow cylinder. Either the youngest, first formed tracheids were g^ushed asunder by 
the centrifugal pressure of the growing and multiplying cells of the young medulla 
developing in their midst (a schizogenetic process), or they were ahsorhed (lysigene- 
tically) under the influence of the same pressure. In the first case, all the relations 
of contact and propinquity between the primary vessels or tracheids composing the 
bundle must have been subjected to a continued succession of changes ; because not 
only had the primary vessels, &c., to enclose a larger area than previoushq but they 
had to allow the intercalation of a succession of newer additional vessels supplied 
from the investing meristemic cortex. This latter necessity is demonstrated by the 
fact that, as the vascular ring increased in diameter, enclosing at the same time a 
growing medulla, the actual number of its component vessels likewise increased. 
My present conviction is that the schizogenetic hypothesis is most in harmony with 
the known facts ; but, should further investigations fail to support this conclusion, 
which I scarcely conceive to be possible, we must then fall back upon the second 
hypothesis. In doing this we must conclude that cdl the vessels or tracheids seen in 
fig. 8 were doomed to undergo absorption, and thus make room for the young medulla 
of fig. 9, and that, in like manner, the successive vascular rings of figs. 10-12 had but 
a temporary existence ; the only permanent vessels being those of fig. 15, h, after the 
formation of which the development of the exogenous zone, d, always appears to have 
arrested the farther expansion alike of the medulla and of its investing vascular 
cylinder in these Lepidodendroid plants.* The objection, in the present case, to this 
* A note on p. 4G6 of my Memoir XII. on this subject is, I fear, an erroneous one. 
