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. [Illustration: 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. [Illustration: Fig. 81.—_Heterangium_ A, Half of the stele of a stem, showing the central mass of wood S mixed with parenchyma _p_. The protoxylem groups _p. x._ lie towards the outside of the stele. Surrounding it is the narrow zone of small-celled secondary wood W. B, A few of the wood cells in longitudinal view: _p. x._, Protoxylem; _p_, parenchyma. S, Large vessels with rows of bordered pits.] The foliage of _Heterangium_ was fernlike, with much-divided leaves similar to those of _Lyginodendron_. We have reason to suspect, though actual proof is wanting as yet, that small Gymnosperm-like seeds were borne directly on these leaves.