The further question as to the relation of these Palaeozoic genera to plants higher in the scale must be reserved for fuller consideration in another volume. An attempt will also be made to consider how far anatomical structure may be used as a guide to the conditions under which Lepidodendron and Sigillaria as well as other members of the Permo-Carboniferous floras passed their lives. The secondary xylem of Lepidodendron and Sigillaria affords a striking example of water-conducting tissue of homogeneous structure comparable with the wood of Conifers rather than with that of Angiosperms. It was presumably formed, for the most part, under uniform climatic conditions: the absence of rings of growth points to uninterrupted supply to evergreen shoots exposed to no alternation of activity and arrested growth. Attention has already been called to the absence of any tissue corresponding to secondary phloem. Even in young shoots of Lepidodendron, no tissue has been found external to the meristematic zone agreeing in the form of its elements with the channels through which the elaborated food is conveyed from the leaves of recent plants to the regions of cell-building. That the ‘secretory zone’ may have served this purpose, at least in young stems, is not improbable. On the other hand, it is difficult to understand why older Lepidodendron stems show no indication of additions to the secretory zone. If this tissue served for the transport of proteids we should expect to find provision made for its constant renewal pari passu with the secondary growth of the xylem. The conclusion seems to me inevitable that the supply of building-material was otherwise provided for than in recent vascular plants. The physiological division of labour may have been less complete in the tissue-systems of the Palaeozoic Lycopods than in the more highly specialised organs of such an extinct genus as Lyginodendron or than in recent plants. Our knowledge of the anatomical structure of many extinct types has already reached a stage when we should take greater heed of the modus operandi of the complex machinery revealed by a study of petrified stems. From the known we proceed to interpret the unknown; but there is a danger of neglecting the possibilities of evolution during the countless ages which separate the forests of the Coal period from those of the present era. We may easily allow preconceived ideas to warp our judgment in attempting to distribute the manifold activities which made up the life of a Lepidodendron among the structural units of the plant-body.
CHAPTER XIX.
Seed-bearing plants closely allied to members of the Lycopodiales.
i. Lepidocarpon.
In 1877 Williamson[662] published an account of some fossil seeds which he referred to Brongniart’s genus Cardiocarpon[663], a generic title for certain Gymnospermous seeds. Some of these he identified, on the authority of the author of the species, with Cardiocarpon anomalum Carruthers[664]. Several years later Wild and Lomax described a new type of strobilus from the Lower Coal-Measures of Lancashire[665]. The result of this discovery and of the subsequent examination by Scott of additional material, was to establish the fact that the seeds described by Williamson and generally accepted as Gymnospermous, are in reality sporangia belonging to a Lycopodiaceous cone. The seeds to which Carruthers gave the name Cardiocarpon anomalum are, however, distinct from those described under the same name by Williamson and are those of a true Gymnosperm. For this seed-bearing strobilus Scott[666] instituted the generic name Lepidocarpon, which he thus defined: “Strobili, with the characters of Lepidostrobus, but each megasporangium was inclosed, when mature, in an integument, growing up from the superior face of the sporophyll-pedicel. Integument, together with the lamina of the sporophyll, completely enveloping the megasporangium, or nucellus, leaving only an elongated, slit-like micropyle above. A single functional megaspore or embryo-sac developed in each megasporangium, occupying almost the whole of its cavity. Megaspore ultimately filled by the prothallus or endosperm. Sporophyll, together with the integumented megasporangium and its contents, detached entire from the axis of the strobilus, the whole forming a closed, seed-like, reproductive body. Seed-like organ horizontally elongated, in the direction of the sporophyll-pedicel, to which the micropylar crevice is parallel.”
Lepidocarpon Lomaxi, Scott. [Fig. 218].
An immature cone of L. Lomaxi is practically identical with a Lepidostrobus; its sporangia are naked and only acquire their integuments at a later stage. A mature strobilus has a diameter of at least 3 cm. and is about 4 cm. in length. As in typical Lepidostrobi, the axis bears spirally disposed sporophylls, and each sporophyll has a long narrow pedicel approximately at right angles to the cone axis with its distal end expanded into a broad and thick lamina ([fig. 218], B).
At the distal end the pedicel has a thin marginal wing ([fig. 218], C, right-hand half) continuous with the upturned protective lamina. To the upper face of each sporophyll is attached along the whole length as far as the ligule, a single large sporangium; on each side of the base of the sporangium the sporophyll forms a supporting cushion. The relation of the sporangium to the ligule, l, is shown in [fig. 218], B, and in the tangential section, C, which illustrates the triangular form of the sporangium near its distal end.
In mature cones, the sporangia assumed the form of seeds, the change being due to the growth of an investing integument from the upper face of the sporophylls on each side of the sporangia. [Fig. 218], A, illustrates the form of a sporangium as shown in tangential sections; the vascular bundle is seen below the base of the sporangium and the gaps right and left of it probably mark the position of parichnos strands. On each side of the sporangium, b, a fairly thick wall of tissue has grown up from the sporophyll, forming an integument which overtops the apical ridge of the sporangium, leaving a narrow micropyle in the form of a long crevice (m, [fig. 218], B). At the proximal end of the sporangium the integument forms an enclosing wall; at the distal end it abuts on and is continuous with the upturned end of the sporophyll. It is clearly established by Scott that the tissue which invests the sporangia is not the upturned margins of the sporophyll, but a new formation fully entitled to the designation integument. It is noteworthy that the integument is not developed until a late stage in the ontogeny of the strobilus; it is not formed until after the production of the prothallus[667]. The diagrammatic sketch, [fig. 218], B, shows the relation of the integument to the sporophyll and sporangium, the outline of the latter being indicated by a broken line. The columnar wall of the sporangium ([fig. 218], A, b) forms a closed beak within the micropylar crevice, and in the interior of the sporangial cavity the slightly shrivelled membrane, a, represents the single megaspore; traces of the aborted sister-cells of the megaspore are occasionally met with. Scott describes a specimen in which the megaspore is filled with tissue agreeing in appearance with the prothallus in a megaspore of Isoetes or Selaginella; no undoubted archegonia or female organs have been discovered, nor has any spore been found containing an embryo.