Most people are acquainted with the horse-tail (equisetum fluviatile) of our marshes and ditches.  It is a somewhat graceful plant, and stands erect with a jointed stem.  The foliage is arranged in whorls around the joints, and, unlike its fossil representatives, its joints are protected by striated sheaths.  The stem of the largest living species rarely exceeds half-an-inch in diameter, whilst that of the calamite attained a thickness of five inches.  But the great point which is noticeable in the fossil calamites and equisetites is that they grew to a far greater height than any similar plant now living, sometimes being as much as eight feet high.  In the nature of their stems, too, they exhibited a more highly organised arrangement than their living representatives, having, according to Dr Williamson, a “fistular pith, an exogenous woody stem, and a thick smooth bark.”  The bark having almost al ways disappeared has left the fluted stem known to us as the calamite.  The foliage consisted of whorls of long narrow leaves, which differed only from the fern asterophyllites in the fact that they were single-nerved.  Sir William Dawson assigns the calamites to four sub-types:  calamite proper, calamopitus, calamodendron, and eucalamodendron.

[Image:  FIG. 7.—­Root of Catamites Suckowii.  Coal-shale.]

[Image:  FIG 8.—­Calamocladus grandis.  Carboniferous sandstone.]

Having used the word “exogenous,” it might be as well to pay a little attention, in passing, to the nomenclature and broad classification of the various kinds of plants.  We shall then doubtless find it far easier thoroughly to understand the position in the scale of organisation to which the coal plants are referable.

[Illustration:  FIG. 9.—­Asterophyllites foliosa.  Coal-measures.]

The plants which are lowest in organisation are known as Cellular.  They are almost entirely composed of numerous cells built up one above the other, and possess none of the higher forms of tissue and organisation which are met with elsewhere.  This division includes the lichens, sea-weeds, confervae (green aquatic scum), fungi (mushrooms, dry-rot), &c.

The division of Vascular plants includes the far larger proportion of vegetation, both living and fossil, and these plants are built up of vessels and tissues of various shapes and character.

All plants are divided into (1) Cryptogams, or Flowerless, such as mosses, ferns, equisetums, and (2) Phanerogams, or Flowering.  Flowering plants are again divided into those with naked seeds, as the conifers and cycads (gymnosperms), and those whose seeds are enclosed in vessels, or ovaries (angiosperms).

Angiosperms are again divided into the monocotyledons, as the palms, and dicotyledons, which include most European trees.

Thus:—­