Equally similar are the cones of the living Equisetum and some of the simple members of the fossil family Calamiteæ, but the more interesting cases are those where differences of an important morphological nature are to be seen.
As regards the second[7] generation there is some very important evidence, from extremely young stages, which has recently been given to the world. In a fern sporangium germinating spores were fossilized so as to show the first divisions of the spore cell. These seem to be identical with the first divisions of some recent ferns (see [fig. 47]). This is not only of interest as showing the close similarity in detail between plants of such widely different ages, but is a remarkable case of delicate preservation of soft and most perishable structures in the “coal balls”.
Fig. 47.—Germinating Fern Spores
A and B, from carboniferous fossils; C, living fern. (A and B after Scott.)
While these few cases illustrate points of likeness between the fructifications of the Coal Measures and of to-day, the large size and successful character of the primitive Coal Measure plants was accompanied by many developments on the part of their reproductive organs which are no longer seen in living forms, and the greater number of palæozoic fructifications must be considered in the next chapter.
CHAPTER VII
MINUTE STRUCTURE OF FOSSIL PLANTS—DIFFERENCES FROM LIVING ONES
We have seen in the last chapter that the main morphological divisions, roots, stems, leaves, and fructifications, were as distinct in the Coal Measure period as they are now. There is one structure, however, found in the Coal Measure fossils, which is hardly paralleled by anything similar in the living plants, and that is the fossil known as Stigmaria. Stigmaria is the name given, not to a distinct species of plant, but to the large rootlike organs which we know to have belonged to all the species of Lepidodendron and of Sigillaria. In the [frontispiece] these organs are well seen, and branch away at the foot of the trunk, spreading horizontally, to all appearance merely large roots. They are especially regularly developed, however, the main trunk giving rise always to four primary branches, these each dividing into two equal branches, and so on—in this they are unlike the usual roots of trees. They bore numerous rootlets, of which we know the structure very well, as they are the commonest of all fossils, but in their internal anatomy the main “roots” had not the structure which is characteristic of roots, but were like stems. In living plants there are many examples of stems which run underground, but they always have at least the rudiments of leaves in the form of scales, while the fossil structures have apparently no trace of even the smallest scales, but bear only rootlets, thus resembling true roots. The questions of morphology these structures raise are too complex to be discussed here, and Stigmaria is only introduced as an example, one of the very few available, of a palæozoic structure which seems to be of a nature not clearly determinable as either root, stem, leaf, or fructification. Among living plants the fine rootlike rhizophores of Selaginella bear some resemblance to Stigmaria in essentials, though so widely different from them in many ways, and they are probably the closest analogy to be found among the plants of to-day.
The individual cells, we have already seen, are strikingly similar in the case of fossil and living plants. There are, of course, specific varieties peculiar to the fossils, of which perhaps the most striking seem to be some forms of hair cells. For example, in a species of fern from the French rocks there were multicellular hairs which looked like little stems of Equisetum owing to regular bands of teeth at the junctions of the cells. These hairs were quite characteristic of the species—but hairs of all sorts have always abounded in variety, so that such distinction has but minor significance.
