No foliage had, however, been met with which was actually attached to the plants, and hence, when it was discovered that some of them had long attenuated leaves not at all like those possessed by ferns, geologists were compelled to abandon this classification of them, and even now no satisfactory reference to existing orders of them has been made, owing to their anomalous structure.  The stems are fluted from base to stem, although this is not so apparent near the base, whilst the raised prominences which now form the cicatrices, are arranged at regular distances within the vertical grooves.

When they have remained standing for some length of time, and the strata have been allowed quietly to accumulate around the trunks, they have escaped compression.  They were evidently, to a great extent, hollow like a reed, so that in those trees which still remain vertical, the interior has become filled up by a coat of sandstone, whilst the bark has become transformed into an envelope of an inch, or half an inch of coal.  But many are found lying in the strata in a horizontal plane.  These have been cast down and covered up by an ever-increasing load of strata, so that the weight has, in the course of time, compressed the tree into simply the thickness of the double bark, that is, of the two opposite sides of the envelope which covered it when living.

Sigillarae grew to a very great height without branching, some specimens having measured from 60 to 70 feet long.  In accordance with their outside markings, certain types are known as syringodendron, favularia, and clathraria. Diploxylon is a term applied to an interior stem referable to this family.

[Illustration:  FIG. 16.—­Stigmaria ficoides.  Coal-shale.]

But the most interesting point about the sigillariae is the root.  This was for a long time regarded as an entirely distinct individual, and the older geologists explained it in their writings as a species of succulent aquatic plant, giving it the name of stigmaria.  They realized the fact that it was almost universally found in those beds which occur immediately beneath the coal seams, but for a long time it did not strike them that it might possibly be the root of a tree.  In an old edition of Lyell’s “Elements of Geology,” utterly unlike existing editions in quality, quantity, or comprehensiveness, after describing it as an extinct species of water-plant, the author hazarded the conjecture that it might ultimately be found to have a connection with some other well-known plant or tree.  It was noticed that above the coal, in the roof, stigmariae were absent, and that the stems of trees which occurred there, had become flattened by the weight of the overlying strata.  The stigmariae on the other hand, abounded in the underclay, as it is called, and were not in any way compressed but retained what appeared to be their natural shape and position.  Hence to explain their appearance,