V. On the organisation of the fossil plants of the coal-measures.—Part XVIII

Abstract

In my last Memoir, Part XVII., I called attention to a spore-bearing strobilus, first described by me, under the name of Volkmannia Dawsoni , in 1871, in the ‘Memoirs of the Literary and Philosophical Society of Manchester.’ This latter description was based upon sections made from a small fragment for which I was indebted to my old auxiliary Mr. John Butterworth, of Shaw, near Oldham, in the autumn of 1870. Beyond two insignificant fragments, seventeen years elapsed before any additional example of this very rare strobilus was discovered. Hence, during that interval, I had no means of confirming, or otherwise, the conclusions arrived at in that early Memoir. Nevertheless, in my Part V. of this series (‘Phil. Trans.,’ 1874) having to deal with some allied forms of Asterophyllitean stems, I again referred to this plant. I pointed out the resemblance between the forms of transverse sections of its central vascular axis ( loc.cit ., Plate 5, figs. 28, c , and 29) and those of the centres of the Asterophyllitean stems figured on Plates I., II., and III. of the same Memoir. The references to this fructification in Part XVII., mentioned above, were connected with my discovery of the vegetative stem of this plant, the structure of which further sustains my conclusions in my Memoir V., not that any specific identity exists, such as I fear some of my expressions in that Memoir might seem to imply, but that in any botanical classification, their positions, though they are generically distinct, must be very near to one another, especially so far as their vegetative organs are concerned. More recently, as stated in a footnote to p. 99 of Part XVII., Mr. Lomax, of Radcliffe, has brought to me a series of specimens which he has discovered and which are of considerable importance, since they make clear a number of features which have hitherto been obscure. On the other hand, several structures of importance, well shown in the specimens figured in my Memoir of 1871, are not preserved in my new examples. Hence, in order that all of what we know of this remarkable plant may be consolidated in the present examination of it, I have reproduced some figures of the more characteristic structures described in 1871. As shewn in Memoir V., fig. 28, transverse sections of the axial vascular bundle of this strobilus have a triangular form, the three projecting angles being broad and abruptly truncated. These features are illustrated by fig. 1, which represents this bundle as seen in the section in my cabinet numbered 1049. The mean diameter of this bundle, measuring from the truncated end of one angle to the more projecting angles of the other two, is about .05 of an inch. The breadth of each angle at its truncated extremity varies from .02 to .03. These measurements approximate closely to what I find in all my specimens, excepting one, of which I have two transverse sections (C. N. 1898, I and L), in which the maximum diameter of the bundle is about ˙0144, and that of the truncate extremity of the angle ˙0032, measurements the proportions of which approximate much more nearly to those of the young twig of Asterophyllites , fig. 1, Memoir V., than is usual in the homologous bundle of Bowmanites . The maximum diameter of the tracheids of this axial bundle, fig. 1, is about .004. None of the other tissues which must once have filled the circular area a" in the centre of which this bundle is placed, are preserved in any of my specimens, with the exception of the narrow line a' of fig. 1, which shows no definite structure; neither have I been able to discover any indications of vascular threads passing outwards from the bundle to the surrounding tissues of the axis; yet it is scarcely to be doubted that some such extensions must have existed. Fig. 1, b , represents a small portion of the innermost surface of what remains of the axial cortex. This organ is seen more perfectly in fig. 2, b (C. N. 1040 B), and a portion of the same is further enlarged in fig. 3. Throughout the greater part of its thickness, this cortex chiefly consists of a rather open parenchyma (fig. 3, b ), but, at its outer border b" , the tissues are more dense and opaque. Longitudinal sections of this stem (fig. 4, b , C. N. 1050) show that these cortical cells are more or less elongated vertically; the outermost of them passing into the prosenchymatous condition shown in fig. 5.