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[13] 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.
CHAPTER XVII
PAST HISTORIES OF PLANT FAMILIES
X. The Lower Plants
In the plant world of to-day there are many families including immense numbers of species whose organization is simpler than that of the groups hitherto considered. Taken all together they form, in fact, a very large proportion of the total number of living species, though the bulk of them are of small size, and many are microscopic.
These “lower plants” include all the mosses, and the flat green liverworts, the lichens, the toadstools, and all the innumerable moulds and parasites causing plant diseases, the green weeds growing in water, and all the seaweeds, large and small, in the sea, the minute green cells growing in crevices of the bark of trees, and all the similar ones living by millions in water. Truly a host of forms with an endless variety of structures.
Yet when we turn to the fossil representatives of this formidable multitude, we find but few. Indeed, of the fossil members of all these groups taken together we know less that is of importance and real interest than we do of any single family of those hitherto considered. The reasons for this dearth of fossils of the lower types are not quite apparent, but one which may have some bearing on it is the difficulty of mineralization. It is self-evident that the more delicate and soft-walled any structure is the less chance has it of being preserved without decay long enough to be fossilized. As will have been understood from Chapter II, even when the process of fossilization took place, geologically speaking, rapidly, it can never have been actually accomplished quickly as compared with the counter processes of decay. Hence all the lower plants, with their soft tissue and lack of wood and strengthening cells, seem on the face of it to stand but little chance of petrifaction.