Fig. 79.—Transverse Section through Petiole of Lyginodendron
v, Fern-like stele; c, cortex; g, glandular hairlike protuberances.
Round the “seed” was a sheath, something like the husk round a hazel nut, which appears to have had the function of a protective organ, though what its real morphological nature may have been is as yet an unsolved problem. On the sheath were glandular hairs like those found on the petiole and leaves, which were, indeed, the first clues that led to the discovery of the connection between the seed and the plant Lyginodendron.
The pollen grains seem to have been produced in sacs very like fern sporangia developed on normal foliage leaves, each grain entered the cavity pc in the seed (see [fig. 56]), but of the nature of the male cell we are ignorant. In none of the fossils has any embryo been found in the “seeds”, so that presumably they ripened, or at least matured their tissues, before fertilization.
These, in a few words, are the essentials of the structures of Lyginodendron. But this plant is only one of a group, and at least two other of the Pteridosperms deserve notice, viz. Medullosa, which is more complex, and Heterangium, which is simpler than the central type.
Fig. 80.—Diagram of Transverse Section of Lagenostoma Seed near the Apex, showing the nine flutings f of the coat c; v, the vascular strand in each; nc, cone of nucellar tissue standing up in the fluted apex of the nucellus n; pc, the pollen chamber with a few pollen grains; s, space between nucellus and coat. Compare with [diagram 56].
Heterangium is found also in rocks rather older than the coal series of England, though of Carboniferous age, viz. in the Calciferous sandstone series of Scotland, it occurs also in the ordinary Coal Measure nodules. It is in several respects more primitive than Lyginodendron, and in particular in the structure of its stele comes nearer to that of ferns. The stele is, in fact, a solid mass of primary wood and wood parenchyma, corresponding in some degree to the protostele of a simple type (see [p. 61], [fig. 36]), but it has towards the outside groups of protoxylem surrounded by wood in both centripetal and centrifugal directions, which are just like the primary bundles in Lyginodendron. Outside the primary mass of wood is a zone of secondary wood, but the quantity is not large in proportion to it (see [fig. 81]), as is common in Lyginodendron.
Though the primary mass is so fernlike in appearance the larger tracheids show series of bordered pits, as do most of the tracheids of the Pteridosperms, in which they show a Gymnosperm-like character.
