328 
PROFESSOR W. WILLIAMSON ON THE ORGANIZATION 
primary vascular wedges at f At to we have a verticil of small lenticular cellular 
areas resembling fig. 23, to, and in the centre of the figure is a large circular area 
representing a branch. In this latter structure we have a medulla composed of 
parenchyma, the central cells of which are larger than the peripheral ones. This 
medulla is encompassed by a vascular zone composed of laminae of vessels arranged 
in slightly curved radiating lines, and which are, beyond all question, derived from 
the surrounding vascular tissues of the parent stem. The continuity between the 
longitudinally intersected vessels of the latter, and the transversely divided ones of 
the branch, is clear and unmistakeable. We further see that the latter evince an 
equally obvious disposition to arrange themselves in numerous small radiating 
vascular wedges, but which exhibit no trace of the large intern odal canals invariably 
found at the inner extremities of the wedges of true Calamitean stems. I am not 
certain that these incipient wedges are identical with the very regular primary 
vascular wedges seen in ordinary Calamites, but they are obviously preparing to 
assume that condition. 
The stem which furnished the last section also supplied me with some others. 
One of these made near the periphery of the vascular zone corresponded very closely 
with fig. 26. On making a second section parallel with the first one, but nearer to 
the medulla, I obtained the result seen in fig. 29. I think there is no room for 
doubting that the lenticular area, to, is the representative of the similar areas 
indicated by the same letter in fig. 23, and that it has been the virtual starting 
point of the young branch ; but we further find some enlarged secondary medullary 
rays, to', to', which may have contributed towards the cellular elements for min g the 
pith of the young growth. This section seems to prove that, as in the higher exogens, 
a connexion is maintained between the medulla of the central stem and that of its 
branches through the intermediate agency of its enlarged medullary rays. 
I think we can have no hesitation in concluding that in the organs just described 
we have the beginnings of peripheral branches before they have emerged from the 
parent stem, and that the specimen shown in fig. 38 of Part I. of these memoirs was, 
as I there suggested, a similar organ. It appears to me further obvious that the organs 
which I there designated infranodal canals, but which it now appears sometimes retain 
their primitive cellular tissues instead of their becoming absorbed, must have fulfilled 
some important function, since, though the large primary medullary rays, in which these 
organs originate, contract their dimensions as they proceed outwards until they become 
virtually merged in the inconspicuous secondary medullary rays of the woody wedges, 
these radial organs not only do not thus disappear, but actually become larger, and their 
boundaries become more clearly defined as they proceed outwards through the vascular 
zone towards the bark. This circumstance explains the great definiteness with which 
we discover the circular and oblong scars marking the position of these radii, even in 
the ordinary specimens of Calamites found imbedded in the coal-shales. 
But we still remain encompassed by physiological and morphological difficulties. Thus 
