The similarity between the Lepidodendron and the modern Lycopod cone has been pointed out already ([p. 67]), and it is this which forms the principal guarantee that they belong to the same family, though the size and wood development of the palæozoic and the modern plants differ so greatly.
The large group of the Lepidodendra included some members whose fructifications had advanced so far beyond the simple sporangial cones described above as to approach very closely to seeds in their construction. This type was described on [p. 75], [fig. 54], in a series of female fructifications, so that its essential structure need not be recapitulated.
Fig. 99.—Transverse Section through Cone of Lepidodendron
A, Main axis with woody tissue; st, stalks of sporophylls cut in oblique longitudinal direction; s, tips of sporophylls cut across; S, sporangia with a few groups of spores. (Microphoto.)
The section shown in [fig. 100] is that cut at right angles to that in which the sporangia are shown in [fig. 98], viz. tangential to the axis. A remarkable feature of the plant is that there were also round those sporangia which bore the numerous small spores (corresponding to pollen grains) enclosing integument-like flaps similar to those shown in [fig. 100], sp. f.
Fig. 100.—Section through one Sporangium of Lepidocarpon
sp, Sporophyll; sp.f., flaps of sporophyll protecting sporangium; S, large spore within the sporangium wall w; s, the three aborted spores of the tetrad to which S belongs.
This type of fructification is the nearest approach to seed and pollen grains reached by any of the Pteridophytes, and its appearance at a time when the Lycopods were one of the dominant families is suggestive of the effect that such a position has on the families occupying it, however lowly they may be. The simple Pteridophyte Lycopods had not only the tall trunks and solid woody structure of a modern tree, but also a semblance of its seeds. Whether this line of development ever led on to any of the higher families is still uncertain. The feeling of most specialists is that it did not; but there are not wanting men who support the view that the lycopod affinity evolved in time and entered the ranks of the higher plants, and indeed there are many points of superficial likeness between the palæozoic Lycopods and the Coniferæ. Judged from their internal structure, however, the series through the ferns and Pteridosperms leads much more convincingly to the seed plants.