ORGANIZATION OF THE FOSSIL PLANTS OF THE COAIL-MEASURES. 877 
primary or as mainly secondary. No sharp line can be drawn between the short and 
somewhat irregularly arranged trachez of the nodal wood, and the longer elements 
with definite radial seriation, which the cambium has added on their outer side. We 
prefer, however, to regard the short-celled wood of the node as primary, and that for 
two reasons: (1) in very favourable preparations (see, for example, Plate 78, fig. 10) 
we see that the nodal wood is continuous with the primary wood of the adjoining 
internode ; (2) in recent Hquiseta an exactly similar mass of wood is developed at the 
nodes, as we have seen especially well in preparations of £. pratense.* Additional 
evidence in favour of the primary nature of the nodal xylem is afforded by the 
anatomy of the axis of the strobilus in the Calamitean fructification described in the 
previous memoir.t Here the tissue in question is fully formed in an organ which is 
without secondary thickening. At the node the trachee are relatively short, and 
have a more or less oblique position with reference to both the radial and tangential 
planes. They may even be nearly horizontal, especially in the neighbourhood of the 
outgoing foliar bundle. 
The cell-walls of the nodal trachez are often reticulated or pitted, while those of the 
adjacent elongated tracheze of the internode are scalariform. Sometimes this difference 
is very conspicuous. Some of the pith-cells bordering on the wood of the node are 
also conspicuously pitted. 
As regards the mode of exit of the foliar bundles we already know that they bend 
out almost at a right angle with the axis (see Plate 72, photographs 5 and 6; Plate 77, 
fig. 6; Plate 78, figs. 7 and 8). Our knowledge of their structure is limited to their 
xylem, for we have no specimens to show the leaf-trace bundles passing through the 
cortex, where alone their phloém could be shown. Their trachez often have a typical 
spiral thickening ; sometimes their walls are reticulated or scalariform (figs. 6 and 7). 
They are accompanied on their outward course by some parenchymatous elements. 
As already mentioned, the xylem of the foliar-bundle is continuous with the proto- 
xylem of the stem-bundle which forms its downward prolongation. 
It is evident that during the formation of secondary wood the elements of the foliar- 
bundle must have been subject to tension and consequent rupture. In spite of this 
they can be traced for a long distance through the wood of old stems, especially in 
tangential sections, though they become very hard to find in the outer layers. It is 
possible that elements may have been added to them for some time, by the cambium.{ 
Apart from the irregularities of distribution already sufficiently dwelt upon, we may 
say that the structure and arrangement of the primary vascular bundles in Calamites 
present an agreement with those of Equisetum, which could scarcely be closer than it is. 
We have for the present left the lateral branches out of account, although they 
* Mr. Cormack regards the nodal wood of Equisetwm as being itself partly secondary. See his paper 
above cited. We do not think this view will hold good for the whole genus. 
+ Witwiamson, “ Organization,” Part XIV., ‘ Phil. Trans.,’ vol. 179, B., 1888, Plate 10, fig. 5. 
¢ Of. SrRAsBURGER, ‘ Histologische Beitriige,’ vol. 3, p. 121. 
