Indeed, so far as fossil evidence goes, we must suppose that the Cycads, since their appearance, possibly at the close of the Palæozoic, have never been a dominant or very extensive family, though they grew in the past all over the world, and in Europe seem to have remained till the middle of the Tertiary epoch.
CHAPTER XII
PAST HISTORIES OF PLANT FAMILIES
V. Pteridosperms
This group consists entirely of plants which are extinct, and which were in the height of their development in the Coal Measure period. As a group they are the most recently discovered in the plant world, and but a few years ago the name “Pteridosperm” was unknown. They form, however, both one of the most interesting of plant families and one of the most numerous of those which flourished in the Carboniferous period.
To mention first the vital point of interest in their structure, they show leaves which in all respects appear like ordinary foliage leaves, and yet bear seeds. These leaves, which we now know bore the seeds, had long been considered as typical fern leaves, and had been named and described as fern leaves. There are two extremely important results from the discovery of this fossil group, viz. that leaves, to all appearance like ordinary foliage, can directly bear seeds, and that the leaves, though like fern leaves, bore seeds like those of a Cycad.
As the name Pteridosperm indicates, the group is a link between the ferns and the seed-bearing plants, and as such is of special interest and value to botanists.
The gradual recognition of this group from among the numerous plant fragments of Palæozoic age is one of the most interesting of the accumulative discoveries of fossil botany. Ever since fossil remains attracted the attention of enquiring minds many “ferns” have been recognized among the rich impressions of the Coal Measures. Most of them, however, were not connected with any structural material, and were given many different names of specific value. So numerous were these fern “species” that it was supposed that in the Coal Measure period the ferns must have been the dominant class, and it is often spoken of even yet as the “Age of Ferns”. From the rocks of the same age, preserved with their microscopical structure perfect, were stems which were called Lyginodendron. In the coal balls associated with these stems (which were the commonest of the stems so preserved) were also roots, petioles, and leaflets, but they were isolated, like the most of the fragments in a coal ball, and to each was given its name, with no thought of the various fragments having any connection with each other. Gradually, however, various fragments from the coal balls had been recognized as belonging together; one specimen of a petiole attached to a stem sufficed to prove that all the scattered petioles of the same type belonged also to that kind of stem, and when leaves were found attached to an isolated fragment of the petiole, the chain of proof was complete that the leaves belonged to the stem, and so on. By a series of lengthy and painstaking investigations all the parts of the plant now called Lyginodendron have been brought together, and the impressions of its leaves have been connected with it, these being of the fernlike type so long called Sphenopteris, illustrated in [fig. 77].
Fig. 77.—Sphenopteris Leaf Impression, the fernlike foliage of Lyginodendron
Fig. 78A.—Diagram of the Transverse Section of Stem of the Lyginodendron