The occurrence of long simple or branched and sometimes capitate hairs is a common feature of Carboniferous megaspores ([fig. 191], E, F, I). It is possible that these appendages served to catch the microspores, thus facilitating fertilisation. A peculiar form of megaspore has been described by Mrs Scott[437], and assigned by her to Lepidostrobus foliaceus, the megasporangium of which apparently contained only four spores. As shown in [fig. 191], G, a large bladder-like appendage characterised by radiating veins is attached to the thick spore-coat; it is suggested that this excrescence may be compared with the “swimming” apparatus of the recent water-fern Azolla. The epithet swimming which it is customary to apply to the appendages of Azolla megaspores would seem to be inappropriate if Campbell[438] is correct in stating that spores of Azolla are incapable of floating.
B. Spencerites.
Spencerites insignis (Williamson). Fig. 192.
- 1878. Lepidostrobus sp., Williamson, Phil. Trans. R. Soc., p. 340, Pl. XXII.
- 1880. Lepidostrobus insignis, Williamson, Phil. Trans. R. Soc., p. 502, Pl. XV. figs. 8–12.
- 1889. Lepidodendron Spenceri, Williamson, Phil. Trans. R. Soc., p. 199, Pl. VII. figs. 20–22; Pl. VIII. fig. 19.
- 1897. Spencerites insignis, Scott, Phil. Trans. p. 83, Pls. XII–XV.
Another type of lycopodiaceous strobilus, differing sufficiently from Lepidostrobus to deserve a special generic designation, is that originally described by Williamson[439], from the Lower Coal-Measures of Yorkshire, as a type of Lepidostrobus, L. insignis, but afterwards[440] more fully investigated and assigned to a new genus by Scott[441]. It should be pointed out that in a later publication Williamson spoke of the lycopodiaceous axis, which he suspected might belong to his L. insignis, as possibly worthy of recognition as a distinct generic type.
Fig. 192. Spencerites insignis (Williamson). (After Miss Berridge.)
Of the two species included by Scott in his genus Spencerites only one, S. insignis, need be considered. Since the publication of Scott’s paper our knowledge of this type has been extended by Miss Berridge[442] and by Prof. Lang[443].
The axis of the strobilus has a stele characterised by a pith of elongated elements, most of which have thin walls; the xylem cylinder possesses about twenty protoxylem strands forming more or less prominent exarch ridges. The cortex exhibits a differentiation comparable with that in the shoots of Lepidodendron. The sporophylls are arranged in alternating verticils, each whorl consisting of ten members: the narrow horizontal pedicel of a sporophyll, containing a single vascular bundle, as shown in [fig. 192], is expanded distally into a prominent upper lobe bearing a cushion of small and delicate cells, to which the sporangium is attached, and prolonged obliquely upwards as a free leaf-like lamina. The lower blunt prolongation of the sporophylls appears to form a thick dorsal lobe, but, as Lang has pointed out, it is highly probable that the present form of the dorsal lobe is of secondary origin, and is “due to the disappearance of a mucilage cavity from a large sporophyll base[444].” As Miss Berridge remarks, the vascular bundle of the sporophyll does not give off a branch to the ventral lobe and sporangium. In attachment, in shape, and in the structure of the wall the sporangia differ markedly from those of Lepidostrobi. The spores, which also constitute a characteristic feature of the genus, have a maximum diameter of 0·14 mm.; they are described as oblate spheroids with a broad hollow wing running round the equator ([fig. 192]) comparable with the air-sacs of the pollen of Pinus. Scott points out that the spores of Spencerites are intermediate in size between the microspores of Lepidodendron and the megaspores of Lycopodium; it is difficult therefore to decide to which category they should be referred. Spencerites is clearly distinct from Lepidostrobus; the absence of a ligule, the manner of attachment of the sporangia, and the form and size of the spores, are characteristic features.
A comparison of Spencerites with the strobili of Lycopodium cernuum (figs. [123], [126–129]) has recently been made by Lang, who draws attention to the striking agreement as regards general plan and even detailed structural features between the Palaeozoic and the recent type of strobilus. It is interesting to find, as Lang points out, that in the original account of the fossil cone by Williamson, the view is expressed that the sporangiophores were confluent. An examination of the section figured by Williamson[445] led Lang to confirm this opinion. It would be out of place to enter here into a detailed comparison of Spencerites insignis and the cone of Lycopodium, but the resemblances are considered by Lang to be sufficiently close to suggest that the striking similarity may be indicative of relationship[446].