Monthly Microscopican 
Journal, Aug. 1, 1869. J 
some Exogenous Stems. 
71 
in the form of a delicate spiral thread winding round the tube at 
wide intervals. In many of the Thujas and other Cupressin£e, an 
immense number of such threads line the previously thickened 
tube, and, running in opposite directions, cover its interior with a 
fine network. Fig. 9 represents a fibre of this kind from Thuja 
Donniana* A further advance is shown in an example of 
Araucaria imhricata which I examined some years ago, and of 
which I fortunately preserved a section, since I have never since 
been able to detect the same structure in other plants of this 
species. Many of the fibres exhibit the appearance represented 
in Fig. 8, a reticular deposit of translucent lignine lining their 
interior, closely corresponding with that of Dictyoxylon. This 
fact appears to me to have some significance in relation to the 
affinities of the Dictyoxylons ; since it is the only instance in which 
I have detected a similar structure in a living conifer. Why I 
fail to find the same structure in other examples of this species I 
cannot discover. Having made the preparation myself, I know it 
to belong to the plant in question ; besides which, its other fea- 
tures are identical with those of corresponding sections of this 
Araucaria which I have made more recently. Be the cause of 
this anomaly what it may, the specimen remains to demonstrate 
that, under some conditions, the ligneous deposits of the conifera 
assume a reticulate form. All the known modifications of reticulate 
vessels were long ago shown by Meyen to be modifications of the 
ordinary spiral vessels — a fact of which the fossil-plants of the 
Coal-measures afibrd ample proof. Figs. 10 and 11 represent a 
portion of a scalariform vessel from a Lepidodendroid plant from 
the Coal-measures, probably a Lomatophloros.t We here have a 
condition intermediate between the simple scalariform vessel and 
the reticulate one of Dictyoxylon. In this example there is a pecu- 
liar feature which I have observed both in young twigs and in old 
stems of the plant, viz. a number of delicate vertical lines of lignine, 
usually simple, less frequently branched, connecting together the 
contiguous transverse bars of the vessel. Fig. 12 represents a few 
of these vertical bands further enlarged. I am not aware that this 
very curious modification of the scalariform vessel has been hitherto 
noticed. 
The examples quoted suffice to show how variable are the 
modifications of the elementary spiral fibre amongst living conifers^ 
and the reticulations of the vessels of Dadoxylon present but one 
more variation of the same primary type. From this we may 
conclude that Dadoxylon and Dictyoxylon separately possess the 
* I have found the same structure in some fossil stems from the Cretaceous 
beds of Missouri. 
t This plant is evidently identical with that described by Mr. Binney in the 
' Phil. Trans.,' under the name of Sigillaria vascularis. I am convinced that it 
was not a Sigillaria, but a form allied to Lepidodendron, as stated above. 
