PAST HISTORIES OF PLANT FAMILIES IX. Sphenophyllales
The group to which Sphenophyllum belongs is of considerable interest and importance, and is, further, one of those extinct families whose very existence would never have been suspected had it not been discovered by fossil botanists. Not only is the family as a whole extinct, it also shows features in its anatomy which are not to be paralleled among living stems. Sphenophyllum became extinct in the Palæozoic period, but its interest is very real and living to-day, and in the peculiar features of its structure we see the first clue that suggests a common ancestor for the still living groups of Lycopods and Equisetaceæ, which now stand so isolated and far apart.
Before, however, we can consider the affinities of the group, we must describe the structure of a typical plant belonging to it. The genus Sphenophyllum includes several species (for which there are no common English names, as they are only known to science) whose differences are of less importance than their points of similarity, so that one species only, S. plurifoliatum, will be described.
We have a general knowledge of the external appearance of Sphenophyllum from the numerous impressions of leaves attached to twigs which are found in the rocks of the Carboniferous period. These impressions present a good deal of variety, but all have rather delicate stems with whorls of leaves attached at regular intervals. The specimens are generally easy to recognize from the shape of the leaves, which are like broad wedges attached at the point. In some cases the leaves are more finely divided and less fanlike, and it may even happen that on the same branch some may be wedge-shaped like those in fig. 112, and others almost hairlike. This naturally suggests comparison with water plants, which have finely divided submerged leaves and expanded aerial ones. In the case of Sphenophyllum, however, the divided leaves sometimes come at the upper ends of the stems, quite near the cones, and so can hardly have been those of a submerged part. The very delicate stems and some points in their internal anatomy suggest that the plant was a trailing creeper which supported itself on the stouter stems of other plants.
The stems were ribbed, but unlike those of the Calamites the ribs ran straight down the stem through the nodes, and did not alternate there, so that the bundles at the node did not branch and fuse as they did in Calamites.
The external appearance of the long slender cones was not unlike that of the Calamite cones, though their internal details showed important distinctions.
In one noticeable external feature the plants differed from those of the last two groups considered, and that was in their size. Palæozoic Lycopods and Equisetaceæ reached the dimensions of great trees, but hitherto no treelike form of Sphenophyllum has been discovered, and in the structure-petrifactions the largest stems we know were less than an inch in diameter.
In the internal anatomy of these stems lies one of the chief interests and peculiarities of the plants. In the very young stage there was a sharply pointed solid triangle of wood in the center, at each of the corners of which was a group of small cells, the protoxylems. The structure of such a stem is like that of a root, in which the primary wood all grows inwards from the protoxylems towards the center, and had we had nothing but these isolated young stems it would have been impossible to recognize their true nature.
Such very young stems are rare, for the development of secondary wood began early, and it soon greatly exceeded the primary wood in amount. Fig. 114 shows a photograph of a stem in which the secondary wood is well developed. The primary triangle of wood is still to be seen in the center, and corresponds to that in fig. 113, while closely fitting to it are the bays of the first-formed secondary wood, which makes the wood mass roughly circular. Outside this the secondary wood forms a regular cylinder round the axis, which shows no sign of annual rings. The cells of the wood are large and approximately square in shape, while at the angles formed at the junction of every four cells is a group of small, thin-walled parenchyma. There are no medullary rays going out radially through the wood, such as are found in all other zones of secondary wood, and in this arrangement of soft tissue the plants are unique.
Beyond the wood was a zone of soft tissue and phloem, which is not often preserved, while outside that was the cork, which added to the cortical tissues as the stem grew.
Petrified material of leaves and roots is rare, and both are chiefly known through the work of the French palæobotanist Renault. The leaves are chiefly remarkable for the bands of sclerized strengthening tissue, and generally had the structure of aerial, not submerged leaves. The roots were simple in structure, and, as in Calamites, had secondary tissue like that in the stems.
In the case of the fructifications it is the English material which has yielded the most illuminating specimens. The cones were long and slender, externally covered by the closely packed tips of the scales, which overlapped deeply. Between the whorls of scales lay the sporangia, attached to their upper sides by slender stalks. A diagram will best explain how they were arranged. Two sporangia were attached to each bract, but their stalks were of different lengths, so that one sporangium lay near the axis and one lay outside it toward the tip of the bract.
In its anatomy the stalk of the cone has certain features similar to those in the stem proper, which were among the first indications that led to the discovery that the cone belonged to Sphenophyllum. There were numerous spores in each of the sporangia, which had coats ornamented with little spines when they were ripe (if examined with a magnifying glass, will show this). Hitherto the only spores known are of uniform size, and there is no evidence that there was any differentiation into small (male) and large (female) spores such as were found in some of the Lepidodendrons. In this respect Sphenophyllum was less specialized than either Lepidodendron or Calamites.
In the actual sections of Sphenophyllum cones the numerous sporangia seem massed together in confusion, but usually some are cut so as to show the attachment of the stalk, as in fig. 117, st. As the stalk was long and slender, but a short length of it is usually cut through in any one section, and to realize their mode of attachment to the axis (as shown in fig. 116) it is necessary to study a series of sections.
Of the other plants belonging to the group, Bowmanites Römeri is specially interesting. Its sporangia were borne on stalks similar to those of Sphenophyllum, but each stalk had two sporangia attached to it. Two sporangia are also borne on each stalk in S. fertile. These plants help in elucidating the nature of the stalked sporangia of Sphenophyllum, for they seem to indicate a direct comparison between them and the sporophylls of the Equisetales.
There is, further, another plant, of which we only know the cone, of still greater importance. This cone (Cheirostrobus) is, however, so complex that it would take far too much space to describe it in detail. Even a diagram of its arrangements is extraordinarily elaborate. To the specialist the cone is peculiarly fascinating, for its very complexity gives him great scope for weaving theories about it; but for our purposes most of these are too abstruse.
Its most important features are the following. Round the axis were series of scales, twelve in each whorl, and each scale was divided into an upper and a lower portion, each of which again divided into three lobes. The lower three of each of these scale groups were sterile and bractlike, comparable, perhaps, with the bracts in fig. 116; while the upper three divisions were stalks round each of which were four sporangia. Each sporophyll segment thus resembled the sporophyll of Calamites, while the long sausage-shaped sporangia themselves were more like those of Lepidodendron. In fig. 118 is a diagram of a trilobed bract with its three attached sporophylls. Round the axis were very numerous whorls of such bracts, and as the cone was large there were enormous numbers of spore sacs.
A point of interest is the character of the wood of the main axis, which is similar to that of Lepidodendron in many respects, being a ring of centripetally developed wood with twelve projecting external points of protoxylem.
This cone is the most complex fructification of any of the known Pteridophytes, whether living or fossil, which alone ensures it a special importance, though for our purpose the mixed affinities it shows are of greater interest.
To mention some of its characters::The individual segments of the sporophylls, each bearing four sporangia, are comparable with those of Calamites, while the individual sporangia and the length of the sporophyll stalk are similar in appearance to those of Lepidodendron. The wood of the main axis also resembles that of a typical Lepidodendron. The way the vascular bundles of the bract pass out from the axis, and the way the stalks bearing the sporangia are attached to the sterile part of the bracts, are like the corresponding features in Sphenophyllum, and still more like Bowmanites.
Many other points of comparison are to be found in these plants, but without going into further detail enough has been indicated to support the conclusion that Cheirostrobus is a very important clue to the affinities of the Sphenophyllales and early Pteridophytes. It is indeed considered to have belonged to an ancient stock of plants, from which the Equisetaceæ, and Sphenophylla, and possibly also the Lycopods all sprang.
Sphenophyllum, Bowmanites, and Cheirostrobus, a series of forms that became extinct in the Palæozoic, remote in their structure from any living types, whose existence would have been entirely unsuspected but for the work of fossil botany, are yet the clues which have led to a partial solution of the mysteries surrounding the present-day Lycopods and Equisetums, and which help to bridge the chasm between these remote and degenerate families.