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.
Medullosa has been mentioned already (see [p. 72]) because of the interesting and unusually complex type of its vascular anatomy. Each individual stele of the group of three in the stem, however, is essentially similar to the stele of a Heterangium.
Though the whole arrangement appears to differ so widely from other stems in the plant world, careful comparison with young stages of recent Cycads has indicated a possible remote connection with that group, while in the primary arrangements of the protosteles a likeness may be traced to the ferns. The roots, even in their primary tissues, were like those of Gymnosperms, but the foliage with its compound leaves was quite fern-like externally. A small part of a leaf is shown in [fig. 83], and is clearly like a fern in superficial appearance. The leaves were large, and the leaf bases strong and well supplied with very numerous branching vascular bundles.
Fig. 82.—Steles of Medullosa in Cross Section of the Stem
A, Primary solid wood; S, surrounding secondary wood.
Fig. 83.—Part of a Leaf of Medullosa, known as Alethopteris, for long supposed to be a Fern