Fig. 218. Lepidocarpon Lomaxi, Scott.

The axis of L. Lomaxi has a medullated stele constructed on the same plan as that of some species of Lepidodendron and Lepidostrobus; the vascular bundles supplying the sporophylls pass obliquely upwards and outwards from the stele, St, fig. 218, B, and bend slightly downward just before entering the pedicel of a sporophyll.

Dr Scott has also described a strobilus containing microsporangia partially enclosed by a rudimentary integument. It is, however, of considerable interest to find a partial development in the case of a male flower of an integumentary outgrowth, which it would seem could only be of real functional importance in the female shoot.

It is important to notice that specimens of a second species of Lepidocarpon, L. Wildianum, are recorded from Lower Carboniferous beds of Scotland, a fact which points to a considerable antiquity for this seed-bearing Lycopodiaceous type[668].

The most important question to consider in regard to Lepidocarpon is—are we justified in applying to the integumented sporangia the term seed? The megaspore was not set free as it is in recent Pteridophytes, such as Azolla and other genera with which Lepidocarpon may be compared; it was on the other hand retained in the sporangium, as may sometimes happen even in recent species of Selaginella (cf. [fig. 131], D). Moreover, the megaspore is characterised by a thin enclosing membrane in contrast to the thick coat of a spore which is destined to be shed. The peculiar slit-like form of the micropyle is a distinguishing feature, but this may be readily explained as a convenient form in the case of a radially elongated sporangium. The absence of an embryo, though a distinguishing feature of Lepidocarpon, cannot be held to be a serious obstacle to the use of the term seed; in recent Cycads the embryo, as Scott points out, may not begin to develope until the seed has been shed. It is possible that the seeds of Lepidocarpon were not pollinated on the parent plant.

The lesson which this extinct type teaches, is that certain Lycopodiaceous plants of the Palaeozoic era had reached an important stage in the evolution of a seed. The morphological essentials of true seeds had been acquired; but we do not know the biological conditions under which pollination and fertilisation were effected. Another point of considerable interest is the value of this discovery as an argument in favour of the view that some Gymnosperms are derived from Lycopod ancestors. Leaving the general question until later, it may at any rate be stated that in Lepidocarpon we have a demonstration of the fact that the Lycopodiales were not always distinguished from Gymnosperms by the absence of seeds. There are certain features in Lepidocarpon shared by the seeds of Araucarieae[669] which may well mean something more than mere parallel development in two distinct phyla of the plant-kingdom[670].

ii. Miadesmia.

In 1894 Prof. Bertrand[671] published an account of certain fragments of petrified leaves and twigs of a small herbaceous Lycopodiaceous plant, under the name Miadesmia membranacea, which he discovered in English material in association with Lepidodendron Harcourtii. Subsequently Scott recognised the megasporophylls of the same plant, and microsporophylls have also been discovered. The most complete account of Miadesmia so far published we owe to Dr Benson[672], whose description is based on specimens from several sources.