12 



NATURE 



[November 3, 1892 



reject Brongniart's idea of its possible affinity to the 

 Marsileae, inclining to the belief that it approached nearer 

 to the Coniferas, and especially to Salisburia. This 

 impression they retained when, at a later date, they 

 described a second species of the same genus. 



In his " Tableau des Genres de Veg^taux Fossiles," 

 published in 1849, Brongniart returns to the subject. He 

 here calls attention to the readiness with which Spheno- 

 phyllum may be confounded with the genus Astero- 

 phyllites, which some forms of the former genus closely 

 resemble ; but he again repeats that the two can be dis- 

 tinguished by the fact that in the former genus the leaves 

 never exceed ten in number, whilst their form is triangular 

 with a truncated summit. He again dwells upon the 

 fact that in some Sphenophylla the leaves become so 

 deeply lobed, narrow, and linear, as to be easily mis- 

 taken for those of Asterophyllites. He now affirms that 

 the fructification is closely related to that of Astero- 

 phyllites. 



As to the affinities of Sphenophyllum, Brongniart now 

 asks, " Does the plant combine the leaves of a Marsilea 

 with the verticillate of an Equisetum, or is it a Gymno- 

 spermous Phanerogam, the leaves of which approach those 

 of the Gingko?" He does not answer the question, but 

 concludes that this cannot be done until the fructification 

 of the plant is better understood. 



In 1864 a monograph on the species of the genus was 

 published by M. Eugene Coemans and M. J. Kickz ; but 

 the authors make no serious effort to solve the vexed 

 question of the affinities of the genus. 



We now enter upon a new stage in the history of the 

 genus. In 1870, M. Renault presented an important 

 memoir to the French Academy of Science, which, for 

 the first time, threw light upon the internal organization, 

 especially of the stems, of Sphenophyllum. He described 

 two examples, one from Autun and the other from St. 

 Etienne, both of which exhibited a structure wholly 

 different from that of any plant previously known, recent 

 or fossil. In the centre of each stem was a primary 

 vascular bundle, the transverse section of which was a 

 triangle with three concave sides and three prolonged, 

 narrow, intermediate arms. This axial organ underwent 

 no subsequent growth after its first formation. But it 

 was invested by a secondary zone, which was deposited 

 upon the primary triangle layer after layer like a 

 secondary xylem, producing a circular axis, which en- 

 larged as the plant advanced in age. But this secondary 

 growth did not consist of layers of vessels, but of 

 vertical columns of thick-walled cubical cells. The 

 cortex also exhibited specially distinctive features. These 

 •discoveries made it clear that Sphenophyllum constituted, 

 not only a very distinct genus, but a type of plant far 

 removed from everything previously described. 



It fell to my lot to make the next advances in our 

 knowledge of this genus. In 1871 I described in the 

 memoirs of the Literary and Philosophical Society of 

 Manchester a new fructification, to which further refer- 

 ence will be made later on. In 1872 I obtained from the 

 Oldham deposits some new stems which obviously be- 

 longed to the same type as those discovered by M. 

 Renault, but from which they differed in important points 

 -of detail. These were described in my Memoir, Part V., 

 published in the Philosophical Transactions for 1874. 

 Transverse sections of these closely resembled in their 

 dominant features M. Renault's corresponding ones, but 

 with two differences. When my plants attained to a 

 certain stage of their exogenous growth, a well-defined 

 circular boundary marked a temporary arrest of that 

 growth, but which started afresh from a zone of much 

 -smaller vessels {loc. cit. PI. II., Figs. 11 and 12), that 

 increased in size as the diameter of the axis in- 

 creased, as they had previously done in the more 

 internal series. Still greater and more important differ- 

 ences presented themselves in the longitudinal sections. 



NO. I 20 J, VOL. 47] 



The zones of secondary or exogenously developed xylem, 

 which in M. Renault's examples consisted solely of verti- 

 columns of thick-walled, cubical cells, were composed, 

 in mine, of true tracheidal vessels with reticulated (not 

 with bordered pits) walls ; presumably a higher stage of 

 development. Another new and more advanced feature 

 than characterise Renault's cells, seen best in tangential 

 sections of this zone {loc. cit. Fig. 13), was the 

 existence, between contiguous tracheids, of vertical, 

 but interrupted, series of small cells, which I can 

 only regard as rudimentary medullary rays. In the 

 same memoir {loc. cit. PI. IV.) a still more distinct form 

 from the Burntisland deposits in Fifeshire was figured 

 and described. M. Renault and Count Solms Laubach 

 refuse to recognize a Sphenophyllum in this type, but 

 they have not yet convinced me that I am in error on 

 the point. The fact is that, though widely aberrant 

 from the form described above, it scarcely differs 

 more from that form than the latter does from 

 M. Renault's examples. 



But my Oldham specimens raised another debated 

 question. W^hen the Memoir V. was published, all 

 authorities agreed that the maximum number of true 

 leaves in each verticil was ten or twelve ; that, however 

 deeply subdivided, their outline was a sphenoid one, and 

 not linear, and that they were multinerved. But I am 

 still convinced that in my specimens there were more 

 than twenty such leaves ; that they were linear in 

 outline, and had a single median nerve. It followed 

 that, continuing to accept the existing definitions of the 

 genus Sphenophyllum, my plant was Asterophylloid 

 rather than Sphenophylloid. I am now prepared to 

 admit that it is a Sphenophyllum ; but only on the con- 

 dition that we alter our definitions of the latter genus, 

 and admit the possibility that some of the forms may 

 possess twenty or more undivided and linear leaves. 

 The accumulating evidence that the foliage of at least 

 some of the Sphenophylla was dimorphic makes the 

 acceptance of my proposition a matter of necessity. 



Yet more recent researches have revealed new and 

 important facts connected with the history of these 

 plants. I have already alluded to the new fructification 

 which I described in 1871, and to which I gave the name 

 of Volkmannia Dawsoni. M. Renault's memoir already 

 noticed was laid before the French Academy in May 

 1870, and noticed in the Comptes Rendus of that date ; 

 but owing to accidents growing out of the Siege of Paris, 

 it was not published until three years later. Mean- 

 while my memoir on Volkmannia Dawsoni was 

 published, and a copy of it forwarded to M. Brongniart. 

 After giving details of the structure of the strobilus 

 I arrived at the conclusion that " it is the fruit 

 either of Asterophyllites or of Sphenophyllum." 



Two years later M. Renault's memoir of 1870 was 

 combined with a second one on the same subject, and pub- 

 lished. It contained a note by M. Brongniart, referring 

 to my memoir of 187 1, in Avhich note he says, " This work 

 agrees in many important points with the results obtained 

 a year previously by M. Renault, though Mr. Williamson 

 was unacquainted with the article in the Comptes Rendus 

 of May 30, 1870. The fossil plant studied by Mr. William- 

 son, and named by him Volkmannia Dawsoni, doubtless 

 differs, at least specifically, from that described by M. 

 Renault, by the form of the central vascular bundle, and 

 by the absence of the zones of quadrangular cells which 

 surround it in the French specimens ; cells which in 

 consequence of the thickness of their walls would not be 

 readily destroyed."^ 



I M. Brongniart has here failed to comprehend an important point. The 

 cells, the absence of which he notices, really belonged to the secondary 

 xylem of the older stem, which did not become developed in the youngest 

 twigs. But it was only upon these twigs that the fructifications were 

 formed, and of which they were but extensions. Hence their absence was 

 merely a consequence of difference of age, and not a feature of specihc 

 value. 



