PAST HISTORIES OF PLANT FAMILIES V. Pteridosperms
This group consists entirely of plants which are extinct, and which were in the height of their development in the Coal Measure period. As a group they are the most recently discovered in the plant world, and but a few years ago the name “Pteridosperm” was unknown. They form, however, both one of the most interesting of plant families and one of the most numerous of those which flourished in the Carboniferous period.
To mention first the vital point of interest in their structure, they show leaves which in all respects appear like ordinary foliage leaves, and yet bear seeds. These leaves, which we now know bore the seeds, had long been considered as typical fern leaves, and had been named and described as fern leaves. There are two extremely important results from the discovery of this fossil group, viz. that leaves, to all appearance like ordinary foliage, can directly bear seeds, and that the leaves, though like fern leaves, bore seeds like those of a Cycad.
As the name Pteridosperm indicates, the group is a link between the ferns and the seed-bearing plants, and as such is of special interest and value to botanists.
The gradual recognition of this group from among the numerous plant fragments of Palæozoic age is one of the most interesting of the accumulative discoveries of fossil botany. Ever since fossil remains attracted the attention of inquiring minds many “ferns” have been recognized among the rich impressions of the Coal Measures. Most of them, however, were not connected with any structural material, and were given many different names of specific value. So numerous were these fern “species” that it was supposed that in the Coal Measure period the ferns must have been the dominant class, and it is often spoken of even yet as the “Age of Ferns”. From the rocks of the same age, preserved with their microscopical structure perfect, were stems which were called Lyginodendron. In the coal balls associated with these stems (which were the commonest of the stems so preserved) were also roots, petioles, and leaflets, but they were isolated, like the most of the fragments in a coal ball, and to each was given its name, with no thought of the various fragments having any connection with each other. Gradually, however, various fragments from the coal balls had been recognized as belonging together; one specimen of a petiole attached to a stem sufficed to prove that all the scattered petioles of the same type belonged also to that kind of stem, and when leaves were found attached to an isolated fragment of the petiole, the chain of proof was complete that the leaves belonged to the stem, and so on. By a series of lengthy and painstaking investigations all the parts of the plant now called Lyginodendron have been brought together, and the impressions of its leaves have been connected with it, these being of the fernlike type so long called Sphenopteris, illustrated in fig. 77.
The anatomy of the main stem is very suggestive of that of a Cycad. The zones of secondary wood are loosely built, the quantity of soft tissue between the radiating bands of wood, and the size of the pith being large, while from the main axis double strands of wood run out to the leaf base. The primary bundles, however, are not like those of a Cycad stem, but have groups of centripetal wood within the protoxylem, and thus resemble the primary bundles of Poroxylon, which are more primitive in this respect than those of the Cycads.
The roots of Lyginodendron, when young, were like those of the Marattiaceous ferns, their five-rayed mass of wood being characteristic of that family, and different from the type of root found in most other ferns (cf. fig. 78B with fig. 35). Unlike fern roots of any kind, however, they have well-developed zones of secondary wood, in which they approach the Gymnospermic roots.
A further mixture of characters is seen in the vascular bundles of the petioles. A double strand, like that in the lower Gymnosperms, goes off to the leaf base from the main axis, but in the petiole itself the bundle is like a normal fern stele, and shows no characters in transverse section which would separate it from the ferns. Such a petiole is illustrated in fig. 79, with its V-shaped fernlike stele. On the petioles and stems were certain rough, spiny structures of the nature of complex hairs. In some cases they are glandular, as is seen in g in fig. 79, and as they seem to be unique in their appearance they have been of great service in the identification of the various isolated organs of the plant.
As is seen from fig. 77, the leaves were quite fern-like, but in structural specimens they have been found with the characteristic glandular hairs of the plant.
The seeds were so long known under the name of Lagenostoma that they are still called by it, though they have been identified as belonging to Lyginodendron. They were small (about ¼ in. in maximum length) when compared with those of most other plants of the group, or of the Cycads, with which they show considerable affinity. They are too complex to describe fully, and have been mentioned already, so that they will not be described in much detail here. The diagrammatic figure (fig. 56) shows the essential characters of their longitudinal section, and their transverse section, as illustrated in fig. 80, shows the complex and elaborate mechanism of the apex.
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, 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.
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, 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, 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.
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 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.
The connection between this plant and certain large three-ribbed seeds known as Trigonocarpus is strongly suspected, though actual continuity is not yet established in any of the specimens hitherto discovered. These seeds have been mentioned before. They were larger than the other fossil seeds which we have mentioned, and, with their fleshy coat, were similar in general organization to the Cycads, though the fact that the seed coat stood free from the inner tissues right down to the base seems to mark them as being more primitive (cf. fig. 55).
Of impressions of the Pteridosperms the most striking is, perhaps, the foliage known as Neuropteris, to which the large seeds are found actually attached (cf. fig. 85).
Ever-increasing numbers of the “ferns” are being recognized as belonging to the Pteridosperms, but Heterangium, Lyginodendron, and Medullosa form the three principal genera, and are in themselves a series indicating the connection between the fernlike and Cycadean characters.
Before the fructifications were suspected of being seeds the anatomy of these plants was known, and their nature was partly recognized from it alone, though at that time they were supposed to have only fernlike spores.
The very numerous impressions of their fernlike foliage from the Palæozoic rocks indicate that the plants which bore such leaves must have existed at that time in great quantity. They must have been, in fact, one of the dominant types of the vegetation of the period. The recent discovery that so large a proportion of them were not ferns, but were seed-bearing plants, alters the long-established belief that the ferns reached their high-water mark of prosperity in the Coal Measure period. Indeed, the fossils of this age which remain undoubtedly true ferns are far from numerous. It is the seed-bearing Pteridosperms which had their day in Palæozoic times. Whether they led directly on to the Cycads is as yet uncertain, the probability being rather that they and the Cycads sprang from a common stock which had in some measure the tendencies of both groups.
That the Pteridosperms in themselves combined many of the most important features of both Ferns and Gymnosperms is illustrated in the account of them given above, which may be summarized as follows::
Salient Characters of the Pteridosperms
F Primary structure of root.
G Secondary thickening of root.
F In Heterangium and Medullosa the
F solid centripetal primary wood of stele.
G Pits on tracheæ of primary wood.
G Secondary thickening of stem.
G Double leaf trace.
F Fernlike stele in petiole.
F Fernlike leaves.
F Sporangia pollen-sac-like.
F Reproductive organs borne directly on ordinary foliage leaves.
G General organization of the seed.
Thus it can be seen at a glance, without entering into minutiæ, that the characters are divided between the two groups with approximate equality. The connection with Ferns is clear, and the connection with Gymnosperms is clear. The point which is not yet determined, and about which discussion will probably long rage, is the position of this group in the whole scheme of the plant world. Do they stand as a connecting link between the ferns on one hand and the whole train of higher plants on the other, or do they lead so far as the Cycads and there stop?