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