primary phloem can be recognized with certainty in favourable
cases, the question of the formation of secondary phloem by the
cambium is not yet fully cleared up. In the Lepidodendron
fuliginosum of Williamson, shown by its leaf-bases to have been a
Lepidophloios, the secondary wood is very irregular, and consists
largely of parenchyma. The same is the case in Lepidodendron
obovatum, one of the few species in which both external and internal
characters are known. The occurrence of secondary growth in
these plants, demonstrated by Williamson's researches, is a point
of great interest. Some analogy among recent Lycopods is afforded
by the stem of Isoëtes, and by the base of the stem in Selaginella
spinosa; in the fossils the process was of a more normal type, but
some of its details need further investigation. The cortex, often
sharply differentiated into sclerotic and parenchymatous zones, is
bordered externally by the persistent leaf-bases. The development
of periderm was a constant feature, and this tissue attained a great
thickness, consisting chiefly of a phelloderm, produced on the inner
side of the formative layer, and no doubt subserving a mechanical
function.
The structure of a Bothrodendron has recently been investigated and proves to be identical with that of the petrified stem which Williamson named Lepidodendron mundum. The anatomy is of the usual medullate Lepidodendroid type; no secondary growth has yet been detected in the stem.
The most interesting point in the structure of the leaf-base is the presence of a ligule, like that of Isoëtes or Selaginella, which was seated in a deep pit, opening on the upper surface of the cushion, just above the insertion of the lamina. The latter shows marked xerophytic adaptations; the single vascular bundle was surrounded by a sheath of short tracheides, and the stomata were sheltered in two deep furrows of the lower surface.
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| Fig. 10.—Lepidostrobus. Diagram of cone, in longitudinal section. | |
| ax, | Axis, bearing the sporophylls (sph), on each of which a sporangium (sm) is seated. |
| lg, | Ligule. |
The upper sporangia contain numerous microspores; in each of the lower sporangia four megaspores are shown.
The cones of Lepidodendron and its immediate allies are for the most part grouped under the name Lepidostrobus. These cones, varying from an inch to a foot in length, according to the species, were borne either on the ordinary twigs, or, as was conjectured, on the special branches (Ulodendron and Halonia) above referred to. In Ulodendron the large circular, distichously arranged prints were supposed to have been formed by the pressure of the bases of sessile cones, though this interpretation of the scars is open to doubt, and it is now more probable that they bore deciduous vegetative branches; in the Halonial branches characteristic of the genus Lepidophloios the tubercles may perhaps mark the points of insertion of pedunculate strobili. The organization of Lepidostrobus is essentially that of a Lycopodiaceous cone. The axis, which in anatomical structure resembles a vegetative twig, bears numerous spirally arranged sporophylls, each of which carries a single large sporangium on its upper surface (fig. 10). The sporophyll, usually almost horizontal in position, has an upturned lamina beyond the sporangium, and a shorter dorsal lobe, so that the form of the whole is somewhat peltate. A ligule is present immediately below the lamina, its position showing that the whole of the elongated horizontal pedicel on which the sporangium is seated corresponds to the short base of a vegetative leaf. The sporangia, usually of very large size compared with those of most recent Lycopods, have a palisade-like outer wall, and contain either an immense number of minute spores or a very small number of exceedingly large spores (fig. 10). It is very doubtful whether any homosporous Lepidostrobi existed, but there is reason to believe that here, as in the closely allied Lepidocarpon, microsporangia and megasporangia were in some cases borne on different strobili. In other species (e.g. in the cone attributed to the Lower Carboniferous Lepidodendron Veltheimianum) the arrangement was that usual in Selaginella, the microsporangia occurring above and the megasporangia below in the same strobilus (diagram, fig. 10). The genus Spencerites (Lower Coal Measures) differs from Lepidostrobus mainly in the insertion of the sporangium, which, instead of being attached along the whole upper surface of the sporophyll, was connected with an outgrowth on its upper surface by a small neck of tissue towards the distal end. The spores of this genus are curiously winged, and intermediate in size between the micro spores and megaspores of Lepidostrobus; the question of homospory or heterospory is not yet decided. The cones of Bothrodendron and another form named Mesostrobus are in some respects intermediate between Lepidostrobus and Spencerites. A more important deviation from ordinary Lepidostroboid structure is shown by the genus Lepidocarpon, from the English Coal Measures and the Lower Carboniferous of Scotland. In this fructification the organization is at first altogether that of a Lepidostrobus; in each megasporangium, however, only a single megaspore came to maturity, occupying almost the whole of the sporangial cavity (see fig. 12), but accompanied by the remains of its three abortive sister cells. An integument grew up from the superior surface of the sporophyll, completely enveloping the sporangium, except for a narrow crevice left open along the top. In favourable cases the prothallus is found preserved, within the functional megaspore or embryo-sac, and the whole appearance, especially as seen in a section tangential tangential to the strobilus, is then remarkably seed-like (see diagram, fig. 11). The seed-like body was detached as a whole from the cone, and in this condition was known for many years under the name of Cardiocarpon anomalum, having been wrongly identified with a true Gymnospermous seed so named by Carruthers. The analogies with a seed are obvious; the chief difference is in the micropyle, which is not tubular, but forms a long crevice, running in a direction radial to the strobilus. Lepidocarpon affords a striking instance of homoplastic modification, for there is no reason to suppose that the Lycopods were on the line of descent of any existing Spermophyta. In a male cone, probably belonging to Lepidocarpon Lomaxi, the microsporangia are provided with incomplete integuments.
| |
|
Fig. 11.—Lepidocarpon Lomaxii. Diagrammatic section of “seed” in plane tangential to the parent strobilus. | |
| sph, | Sporophyll. |
| vb, | Its vascular bundle. |
| i, | Integument. |
| m, | Micropylar crevice. |
| a, | Base. |
| sm, | Wall of sporangium. |
| mg, | Membrane of functional megaspore, which is filled by the prothallus, pr. |
Another case of a “seed-bearing” Lycopod has lately been discovered by Miss Benson in Miadesmia membranacea, a slender Selaginella-like plant from the Lower Coal Measures of Lancashire. The female fructification is in the form of a rather lax strobilus. Each sporophyll bears a megasporangium, attached to its upper surface at the proximal end, containing a single large megaspore (fig. 13). The megasporangium is enclosed in an integument, which completely envelopes it, leaving only a narrow micropyle at the distal end (fig. 13). The long tentacles of the integument may have served to facilitate pollination. The seed-like character of the organ is even more striking in Miadesmia than in Lepidocarpon. There seems to be no near affinity between these genera, in which the seed-habit must have arisen independently.
| |
| (Scott, Studies.) | |
|
Fig. 12.—Lepidocarpon Lomaxii. Sporangium and sporophyll before development of integument. (× about 12.) | |
| cu, | Lateral cushions on sporophyll. |
| vb, | Vascular bundle, |
| wp, | Palisade layer of sporangium-wall. |
| wi, | Inner layer of wall. |
| a, | Base of sporangium. |
| mg, | Membrane of megaspore or embryo-sac. |
Sigillaria.—The great genus Sigillaria, even richer in “species” than Lepidodendron, ranges throughout the Carboniferous, but has not yet been detected in earlier rocks. The Sigillariae, like the Lepidodendra, were large trees, but must have differed from those of the previous group in habit, for they appear to have branched sparingly or not at all, the lofty upright shaft terminating, like some modern Xanthorrhaea, in a great sheaf of long, grass-like leaves. The strobili were stalked, and borne on the main stem, among the leaves. The roots, or at least their functional representatives, resembled those of Lepidodendron. The chief distinctive character of Sigillaria lies in the arrangement of the leaf-scars, which form conspicuous vertical series on the surface of the stem.
