PERHAPS the most abundant of all Coal Measure plant fossils are the remains of the giant club-mosses. The remarks just made about the horse-tails apply to this group also. The living representatives are small herbaceous plants some of which are popularly known as club-mosses, though
in spite of the name they have nothing to do with true mosses. Their allies (but not actual ancestors) in Carboniferous times were often huge trees over a hundred feet in height, capable of increasing in thickness by forming secondary wood; in many cases their foliage of small densely-crowdedwas similar to that of the club-mosses. It is well to be familiar with two of the principal genera of these trees, Lepidodendron and Sigillaria, for fragments of their , showing impressions of the crowded -bases, lozenge-shaped in the former and more rectangular or hexagonal in the latter, are exceedingly common in the coal-beds. Still commoner, perhaps, is the fossil called Stigmaria, which is now known to be the ramifying underground rootstock of both Sigillaria and Lepidodendron. The finer rootlets penetrated everywhere in the coal swamps, and the coal-balls described above are always full of them.
The cones of these plants produced two kinds of spores, and were so prodigal of their pollen, as the smaller spores may be called, that certain deposits of coal are practically made of nothing else; probably in such cases the pollen drifted into the still waters of a pool and collected at the bottom. The larger spores (also very common in coal, their resistant coats often remaining when the vegetable matter decayed) were sometimes reduced to a single one in each spore-sac. By a further development in which the spore-sac was closely invested by a leafy structure, leaving only a slit for the entrance of the pollen, a-like organ was produced. Other types of have also been found in this group, showing that seed formation was not confined either to the or their ancestors, for the seed-bearing club-mosses belong to an entirely different line of descent which died out long ago.