On Sporocarpon ornalum. 19

Transcription

1 On Sporocarpon ornalum. 19 Ashton-under-Lyne,' showing the existence of a canal in the centre of each of the projections or crenulations, whilst the peripheral clusters of large cells are wanting. The object of the present note is to record the fact that Sporocarpon ornatnm is nothing else than a transverse section of Lagenostoma physoides, a seed of which longitudinal sections had already been described and figured by Williamson in his 8th Memoir.* This identification is based on a comparison of the specimens in the Williamson Collection with others lately received from Mr. James Lomax, of Bolton. The agreement between the transverse and longitudinal sections is convincing in respect of structure, mode of preservation of the various layers, and dimensions. It is intended at some future time to describe the additional features that have been ascertained concerning the structure of this very interesting and little-known seed of the English Coal-Measures. In conclusion the writer would express his indebtedness to Miss Benson of the Royal Holloway College, and to Dr. D. H. Scott, F.R.S.. who have kindly permitted him to examine preparations in their possession. F. W. OLIVER. In the explanation of the figures, loc, cit., p. 474, tbis section is by an oversight named Sporocarpon anomahim. *Pliil. Traus. 1S77, p. 241, and Figs. 77, 78 and 79. PROFESSOR BOMMER ON LEPIDOCARPON. ON the 18th of March last, my friend Professor Charles Bommer gave an account of the new genus Lepidocarpon, before the Belgian Geological Society.' Such a critical discussion of recent work is greatly to be welcomed, especially by the author whose work is discussed ; it is a pleasure to me to find, in this instance, that the views of Professor Bommer, as regards the wider questions involved, are in essential agreement with my own. There are one or two points, however, where his interpretation does not seem to >Le Genre I<epidocarpon, Scott; Bull, de la Soc. Helge de Geologie, t. xvi. 1902, pp. i3»-«37-

2 2O Professor Bommer on Lepidocarpon. me to be entirely in accordance with the observed facts; these points I propose briefly to reconsider here. Lepidocarpon, it will be remembered', is a Lycopodiaceous fructification of Carboniferous age, remarkable for the seed-like character assumed by the mature megasporangia. This character depends chiefly on two facts: 1. That in each sporangium only a single megaspore came to maturity, forming the prothallus within it, and occupying, like an embryo-sac, almost the whole of the sporangial cavity. 2. That an envelope grew up from the sporophyll around the sporangium, completely enclosing it, except for a narrow crevice along the top. Further, all the evidence shows that the megaspore was retained permanently within the seed-like organ, which, like a true seed, was shed as a whole. The chief point to be noticed concerns the nature of the envelope or integument. Professor Bommer says (p. 134); "Les sporanges sont prot^gds par les bords de la sporophylle qui, en se repliant audessus d'eux, leur constituent une enveloppe close et resistante," etc. This interpretation of the integument as consisting of the infolded edges of the sporophyll is one which I have considered in my paper, and rejected, on grounds which Professor Bommer has not dealt with in his review. My reasons were these : 1. In the distal part of the sporophyil the lamina is clearly seen projecting laterally beyond the base of the integument, which cannot therefore be identical with it-; 2 There is good evidence that the integument was closed at its proximal end (towards the axis) a condition which a mere infolding of the margins will obviously not account for.' These important points appear to have escaped Professor Bommer's attention, for he merely refers to the presence of 'Scott, Structure nnd Affinities of Fossil Plants from Palaeozoic Kocks; IV. The seed-like Fructification of Lepidocarpon. Phil. Trans. Roy. Soc. B. vol.. 194, See also NKW PnYTor.OGiST. February 1902, for a review by F. W. O. 'Scott, 1. c, p. 306, PI. \\, Fig. 13. M. c, p. 308, PI. 4^ Fig. 24.

3 Professor Bommer on Lepidocarpon. 21 lateral furrows in the sporophyll of Lepidocarpon Lomaxi, a fact on which I have laid no stress, as it has no evident beai ing on the question as to the nature of the integument. The author considers the species L. Wildianum especially favourable to his interpretation, but I have not found any essential difference between the two species; in L. Wildianum also there is evidence for the presence of a laminar margin distinct from the integument.' On a consideration of all the facts, it seems clear that the integument in the genus Lepidocarpon was a new formation, not represented in the young sporophylls or in those of ordinary Lepidostrobi. Neither is the velum of Isoetes formed by the infolded margins of the sporophyll, as Professor Bommer, no doubt by an oversight, states (p. 135); his own figure shows that this is not the case.- The velum of Isoetes springs partly from the saddle at the distal end of the sporangium, partly from the upper surface of the sporophyll-base, but not from its margins; like the integument of Lepidocarpon it is a new formation an indusium. I agree with Professor Bommer that the integument of a seed is also a sort of indusium (p. 136). The statement that in Lepidocarpon the sporangium is protected by the whole of the sporophyll is true, but with the reservation that this protection is only carried out with the help of a new organ, the integument, specially adapted to this function. That the sporophyll as a whole takes part in the formation of the seed-like organ is a real point of difference from a typical seed, as I had already stated.' In conclusion our author remarks that there are two facts which materially reduce the morphological importance of the integument of the megasporangia in Lepidocarpon (p. 136). One of these facts is the presence of a simiiar protective envelope around the microsporangia. On this point I have nothing to add to what is said in my paper (p. 322). An envelope originally common to both kinds of sporangia may well have been retained and further developed on the female side only, where its functions would obviously be more important. In our fossil there is already a considerable difference between the integunients of the two organs, for that of the microsporangium forms only a very partial investment.* '1. c, p.3i6, PI. 42. Fig. 18.»See also Campbell, Mosses and Ferns, Fig. 144, H- M. c, p 321- "I c, p. 313-

4 2 2 Professor Bommer on Lepidocarpon. Professor Bommer's second point is the fact that so many of the megasporangia are without any integument. I have suggested that these specimens may represent the young stage of the organ in an arrested condition, to which our author objects that the suggested arrest of development only affects the external envelope, the megaspore, which is the essential part, being fully formed. Since my paper was published I have observed a non-integumented sporangium in which the megaspore already contains a prothallus,' a fact which no doubt adds force to Professor Bommer's criticism. Still, I can see no other explanation of the facts than that, from some cause or other (possibly the absence of pollination), certain sporangia were arrested in their development before the integument was formed. That among equally mature organs some should be with and others without such an important appendage, seems highly improbable, though perhaps a remote analogy might be found in the inconstancy of development of the indusium in some Ferns. Although I am not able wholly to accept Professor Bommer's interpretation of the structure of Lepidocarpon,-we are in substantial agreement in our views of the general significance of the type of fructification which it exhibits, and in our estimate of its biological importance. In his concluding paragraph Professor Bommer sums up the question in some admirable remarks with which I am entirely in accord. '* En r^sum^t Lepidocarpon nous offre I'exemple d' un Lycopodin^e tres dvolu^e par l'accentuation d'une serie de dispositions sp^ciales, rdalisdes d^jk isol^ment ou k un moindre degrd chez d'autres Pt^ridophytes, Les charact^res particuliers d'adaptation de son appareil de reproduction offrent, fonctionnellement, un paralldlisme remarquable avec ce qui existe chez les Phan^rogames." D. H. SCOTT. 'This section is in the Collection of Fossil Slides belonging to the Botanical Department of University College. I<ondon.

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