Fig. 92.—Impression of Palæozoic Fern, showing sori on the pinnules. (Photo.)
These two big groups from the Palæozoic include practically all the ferns that then flourished. They have been spoken of (together with a few other types of which little is known) as the Primofilices, a name which emphasizes their primitive characters. As can be seen by the complex organization of the genera, however, they themselves had advanced far beyond their really primitive ancestors. There is clear indication that the Botryopterideæ were in a period of change, what might almost be termed a condition of flux, and that from their central types various families separated and specialized. Behind the Botryopterideæ, however, we have no specimens to show us the connection between them and the simpler groups from which they must have sprung. From a detailed comparative study of plant anatomy we can deduce some of the essential characters of such ancestral plants, but here the realm of fossil botany ceases, to give place to theoretical speculation. As a fact, there is a deep abyss between the ferns and the other families of the Pteridophytes, which is not yet bridged firmly enough for any but specialists, used to the hazardous footing on such structures, to attempt to cross it. Until the buttresses and pillars of the bridge are built of the strong stone of fossil structures we must beware of setting out on what would prove a perilous journey.
In the Coal Measures and previous periods we see the ferns already represented by two large families, differing greatly from each other, and from the main families of modern ferns which sprung at a later date from some stock which we have not yet recognized. But though their past is so obscure, the palæozoic ferns and their allies throw a brilliant light on the course of evolution of the higher groups of plants, and the gulf between ferns and seed-bearing types may be said to be securely bridged by the Botryopterideæ and the Pteridosperms.
CHAPTER XIV
PAST HISTORIES OF PLANT FAMILIES
VII. The Lycopods
The present-day members of this family are not at all impressive, and in their lowliness may well be overlooked by one who is not interested in unpretending plants. The fresh green mosslike Selaginella grown by florists as ornamental borders in greenhouses and the creeping “club moss” twining among the heather on a Highland moor are probably the best known of the living representatives of the Lycopods. In the past the group held a very different position, and in the distant era of the Coal Measures it held a dominant one. Many of the giants of the forest belonged to the family (see [frontispiece]), and the number of species it contained was very great.
Let us turn at once to this halcyon period of the group. The history of the times intervening between it and the present is but the tale of the dying out of the large species, and the gradual shrinking of the family and dwarfing of its representative genera.
It is difficult to give the characters of a scientific family in a few simple words; but perhaps we may describe the living Lycopods as plants with creeping stems which divide and subdivide into two with great regularity, and which bear large numbers of very small pointed leaves closely arranged round the stem. The fruiting organs come at the tips of the branches, and sometimes themselves divide into two, and in these cone-like axes the spore cases are arranged, a single one on the upper side of each of the scales (see [p. 67], [fig. 46], A). In the Lycopods the spores are all alike, in the Selaginellas there are larger spores borne in a small number (four) in some sporangia (see [fig. 53], [p. 75]), and others in large numbers and of smaller size on the scales above them. The stems are all very slender, and have no zones of secondary wood. They generally creep or climb, and from them are put out long structures something like roots in appearance, which are specially modified stem-like organs giving rise to roots.
From the fossils of the Coal Measures Lepidodendron must be chosen as the example for comparison. The different species of this genus are very numerous, and the various fossilized remains of it are among the commonest and best known of palæontological specimens. The huge stems are objects of public interest, and have been preserved in the Victoria Park in Glasgow in their original position in the rocks, apparently as they grew with their spreading rootlike organs running horizontally. A great stump is also preserved in the Manchester Museum, and is figured in the [frontispiece]. While among the casts and impressions the leaf bases of the plant are among the best preserved and the most beautiful (see [fig. 93]). The cone has already been illustrated (see [fig. 46] and [fig. 9]), and is one of the best known of fossil fructifications.
Fig. 93.—Photo of Leaf Bases of Lepidodendron