Fig. 111. Sphenophyllum speciosum (Royle).
A. Nat. size. B. enlarged leaf.
From the Raniganj Coal-field, India. (After Feistmantel.)
The inequality of the members of a single whorl, which characterises this Indian plant, is sometimes met with in European species. A specimen of Sphenophyllum oblongifolium, which Prof. Zeiller showed me in illustration of this point, was practically indistinguishable from Trizygia[921].
In some of the earlier descriptions of the Indian species the generic name Sphenophyllum[922] was used by McClelland and others, but the supposed difference in the leaf-whorls was made the ground of reverting to the distinct generic term Trizygia. Now that a similar type of leaf-whorl is known to occur in Sphenophyllum, it is better to adopt that genus rather than to allow the question of locality to unduly influence the choice of a separate generic name for an Indian plant.
GEOLOGICAL RANGE.
C. Affinities, range and habit of Sphenophyllum.
It has been pointed out in the description of Sphenophyllum, that the most widely separated families of recent plants have been selected by different authors as the nearest living allies of this Palaeozoic genus. It is now generally admitted that Sphenophyllum is a generic type apart; it cannot be classed in any family or sub-class of recent or fossil plants, without considerably extending or modifying the recognised characteristics of existing divisions of the plant-kingdom. The anatomical characters of the Sphenophyllum stem are such as one finds in some recent genera of the Lycopodineae, especially Psilotum. If the stele of Psilotum were composed internally of a solid strand of xylem, we should have a close correspondence between the centripetally-developed wood of this genus and that of Sphenophyllum. Similar comparisons might be drawn with other existing genera, but the more detailed consideration of the affinities of the Palaeozoic plant will be more easily dealt with after other members of the Pteridophytes have been described. The recent discovery of an entirely new type of Carboniferous strobilus in rocks of Calciferous sandstone age on the shores of the Firth of Forth has thrown new light on the position of Sphenophyllum. Cheirostrobus Pettycurensis, the new cone which Scott has described in an able memoir, affords certain points of contact with Sphenophyllum on the one hand and with Calamites on the other. This important question will be dealt with after we have given an account of Cheirostrobus[923]. To put the matter shortly, Sphenophyllum agrees with some Lycopodinous plants in its anatomical features; with the Equisetales it is connected by the verticillate disposition of the leaves, and some of the forms of Sphenophyllum strobili present features which also point to Equisetinous affinities.
In his Presidential address to the Botanical Section at the British Association Meeting of 1896 Scott[924] thus refers to the Sphenophyllums:—“We may hazard the guess that this interesting group may have been derived from some unknown form lying at the root of both Calamites and Lycopods. The existence of the Sphenophyllae certainly suggests the probability of a common origin for these two series.” The result of the subsequent investigation of the new cone Cheirostrobus amply justifies this opinion as to the position of Sphenophyllum.
It is probable that Sphenophyllum lived during the Devonian period, but the unsatisfactory specimens on which Dawson has founded a species of this age, S. antiquum[925], can hardly be said to afford positive evidence of the Pre-Carboniferous existence of the genus. From the Culm rocks and other strata older than the Coal-Measures, we have such species as S. insigne (Will.), Sphenophyllostachys Römeri (Solms-Laubach), and Sphenophyllum tenerrimum, Ett.[926] while S. emarginatum[5], Brongn. occurs in the Upper Coal-Measures and in the Transition rocks. S. cuneifolium[927] (Sternb.) has been recorded from the Transition, Middle and Lower Coal-Measures. Sphenophyllum oblongifolium, Germ.[928], is recorded from Lower Permian rocks, as is also S. Thoni[929], Mahr.
The comparison which has naturally been drawn between Sphenophyllum with its slender stems bearing occasionally dimorphic leaves, and water-plants is not, I believe, supported by the facts of anatomy or external characters. The entire and finely-dissected leaves do not exhibit that regularity of relative disposition which is characteristic of aquatic plants; the two forms of leaves may occur indiscriminately on the same branch. The well-developed and thick xylem is not in accordance with the anatomical features usually associated with water-plants. It is true that in some living dicotyledons of the family Leguminosae, which inhabit swampy places, the secondary xylem is represented by a thick mass of unlignified and thin-walled parenchyma, as in the genus Aeschynomene[930], from which the material of ‘pith’-helmets is obtained; but the wood of Sphenophyllum was obviously thick-walled and thoroughly lignified.
It is not improbable that the long and slender stems of this plant may have grown like small lianas in the Coal-Measure forests, supporting themselves to a large extent on the stouter branches of Calamites and other trees. The anatomical structure of a Sphenophyllum stem would seem to be in accord with the requirements of a climbing plant. It has been shewn[931] that in recent climbing plants the tracheae and sieve-tubes are characterised by their large diameter, a fact which may be correlated with the small diameter of climbing stems and the need for rapid transport of food material. In Sphenophyllum the tracheae of the xylem have a wide bore, and in S. insigne the phloem contains unusually wide sieve-tubes. The central position of the stele is another feature which is not inconsistent with a climbing habit. Schwendener and others[932] have demonstrated that in climbing organs, as in underground stems and roots, there is a tendency towards a centripetal concentration of mechanical or strengthening tissue. The axial xylem strand of Sphenophyllum would afford an efficient resistance to the tension or pulling force which climbing stems encounter.