There can be little doubt that some of the Mesozoic and Tertiary species included under Lycopodites agree more closely with the recent genus Selaginella than with Lycopodium, but this does not constitute an argument of any importance against the restricted use of the designation Selaginellites which we have adopted. From a botanical point of view the various records of Lycopodites and Selaginellites have but a minor importance; they are not sufficiently numerous to throw any light on questions of distribution in former periods, nor is the preservation of the material such as to enable us to compare the fossil with recent types either as regards their anatomy or, except in a few cases, their sporangia and spores. The Palaeozoic species are interesting as revealing less reduction in the number of spores produced in the megasporangia. Among existing Pteridophytes the genus Isoetes agrees more closely than Selaginella, as regards the number of megaspores in each sporangium, with such fossils as Selaginellites Suissei and S. elongatus.
It would seem that in most Palaeozoic species heterospory had not reached the same stage of development as in the recent genus Selaginella in which the megaspores do not exceed four in each sporangium. In Selaginellites primaevus, however, the heterospory appears to be precisely of the same type as in existing species.
Lycostrobus.
The generic name Lycostrobus has recently been instituted by Nathorst[232] for certain specimens of a lycopodiaceous strobilus, from the Rhaetic strata of Scania, which he formerly referred to the genus Androstrobus[233].
Lycostrobus Scotti Nathorst. Fig. 139.
The fossil described under this name is of special interest as affording an example of a Mesozoic lycopodiaceous cone comparable in habit and in size with some of the largest examples of Palaeozoic Lepidostrobi, the cones of Lepidodendron. The Swedish fossil from Upper Rhaetic strata of Helsingborg (Scania) was originally designated Androstrobus Scotti, the generic name being adopted in view of the close resemblance of the form of the strobilus to the male flower of a Cycad. A more complete examination has shown that the bodies, which were thought to be pollen-sacs—though Nathorst recognised certain differences between them and the pollen-sacs of lycopods—are the megaspores of a lycopod. Microspores have also been identified. The axis of the cone has a breadth of 2 cm. with a peduncle which may be naked or provided with a few small scales; the sporophyll region of the axis reached a length of at least 12 cm. The spirally disposed sporophylls terminate in a rhombic distal end which may represent the original termination or they may have been prolonged upwards as free laminae. Each sporophyll bears on its upper face a single large sporangium containing either megaspores or microspores: the megaspores, 0·55–0·60 mm. in diameter, are finely granulate and bear small warty thorns or more slender pointed appendages. The microspores, after treatment with eau de Javelle, were found to measure 36–44μ while others which had been treated with ammonia reached 54μ in diameter. Nathorst describes the microspores as occurring in spherical groups or balls, which it is suggested may be compared with the groups of spores separated by strands of sterile tissue (trabeculae) in the large sporangia of Isoetes (cf. [fig. 133], H). If this comparison is sound it would point to a more complete septation of the sporangium in Lycostrobus than in any recent species of Isoetes. The size of the strobilus would seem to indicate the persistence into the Rhaetic era of an arborescent lycopodiaceous type; but the appearance and manner of preservation of the axis is interpreted by Nathorst as evidence of a herbaceous rather than a woody structure. He is disposed to regard Isoetes as the most nearly allied existing genus.
Fig. 139. Lycostrobus Scotti, Nath. (After Nathorst; ⅘ nat. size.)
The comparison made by Nathorst with Isoetes is based on a resemblance between the spores of the two genera and on the evidence, which is not decisive, of the existence of sterile strands of tissue in the sporangia of Lycostrobus. This similarity is however hardly of sufficient importance to justify the inclusion of the Rhaetic strobilus in the Isoetaceae. In size and in the arrangement and form of the sporophylls the cone presents a much closer resemblance to Lepidodendron than to Isoetes. It is probably advisable to regard this Rhaetic type simply as a lycopodiaceous genus which we are unable, without additional information, to assign to a particular position.
The opinion expressed by Professor Fliche[234] that the plant described by Schimper and Mougeot as Caulopteris tessellata, a supposed tree-fern stem, from Triassic rocks of Lorraine, is more probably a large lycopodiaceous stem, either a Lepidodendron or a new genus, is worthy of note in reference to Nathorst’s account of Lycostrobus.