XIII. On the organization of the fossil plants of the coal-measures. —Part III. Lycopodiaceæ (continued)

Abstract

In the last memoir which I laid before the Royal Society I described a number of forms of Lepidodendroid plants from the Coal-measures, without making any material attempt to ascertain the relationship which they bore to each other. I now propose to carry the subject somewhat further, and to show that some of these apparently varied forms of Lycopodiaceæ merely represent identical or closely allied plants in different stages of their growth. The discovery of some remarkable beds in Burntisland, by George Grieve, Esq., and his persistent kindness in supplying me abundantly with the raw material upon which I could work, have enabled me to do this in a manner, at least, satisfactory to myself. Upon the geology of these remarkable beds I will not now enter, beyond saying that they appear to have been patches of peat belonging to the lower Burdiehouse series, which are now imbedded in masses of volcanic amygdaloid. The stratum, where unaltered by contact with the lava, is little more than a mass of vegetable fragments, the minute structure of most of which is exquisitely preserved. The more perfect remains that are capable of being identified belong to but few types. The most abundant of these are the young twigs of a Lepidodendron , portions of the stem of a Diploxylon , stems of a remarkable Lycopodiaceous plant belonging to my new genus Dictyoxylon (but which, for reasons to be stated in a future memoir, I propose to unite with Corda’s genus Heterangium , under the name of H. Grievii ), and fragments of Stigmaria-ficoides . Along with these occur, but more rarely, several other curious Lycopodiaceous and Fern stems, and those of an articulated plant, which I believe to be an Asterophyllites ; also some true Lepidostrobous fruits and myriads of caudate macrospores belonging to the Lepidostrobi . The first point to be noted is that all the Lepidodendroid branches are young twigs. No one example of a large stem has been found presenting exactly the same structure as these small branches, which, as already stated, are so abundant. On the other hand, all the Diploxylons are large branches or matured stems. These facts at once suggested the inquiry whether the two plants referred to might not be complementary to each other. A careful and very extended study of a large number of specimens has convinced me that such is the case. I have made more than a hundred sections of the two forms, and the result has been a remarkably clear testimony that the Lepidodendra are the twigs and young branches of the Diploxylon -stems. I am also led to the conclusion that the Lepidostrbi , with their peculiar macrospores and microspores, belong to the same plant. I will examine each of these forms in detail.