The Ophioglossaceæ, or Adder’s Tongue ferns, are few in number, and present comparatively little difference in structure. They are known among the exannulate series, from the Marattiaceæ, by having their fertile fronds contracted, bearing their sporangia at the margin, so that being entirely occupied by sporangia, the fertile fronds appear as if they were an inflorescence distinct from the foliaceous organs, as indeed they are analogically. In this group there is the further difference that the fronds are not circinate, but straight, in their æstivation.
These Adder’s Tongue ferns are dwarf herbaceous plants, differing greatly in structure from the true ferns, though there is some similarity to them in the fructification. They are divided into four genera, of which Ophioglossum and Botrychium are best known. The base of the stem is thick and bulbiform, and sends off spreading succulent roots. The species of the genus Ophioglossum are chiefly plants of a warm climate, but Ophioglossum vulgatum, or common Adder’s Tongue, is distributed in almost every part of the globe. In England it is exceedingly abundant in meadows and pastures, and varies in height from a few inches to a foot, in moist soil. It has one barren ovate and one fertile linear frond. The barren frond, which is of a yellowish-green colour, invests the stem of the fertile frond like a spathe; its form is ovate, varying to ovate lanceolate, and more or less obtuse, with a complicated network of anastomosing veins. Bivalved sporangia form two parallel series on the margins of the club-like terminations of the fertile frond. Soon after the fruit is shed the fronds die, but one or two buds are previously formed at the base of the plant, which remain dormant till the following spring.[[74]]
The genus Botrychium has few species, about half of which grow in North America. Botrychium Lunaria, or Moonwort, is the only species indigenous in Great Britain. This plant, which is from five to six inches high, has a rather long succulent stem, invested at the base by the dark-coloured membranous remains of withered leaves. It has one barren frond, the base of whose stem is a sheath, through which the tall, fertile frond rises. The barren frond has from three or four to seven pairs of opposite smooth pinnæ, of a lunate shape and glaucous green colour, overlapping each other; they are smooth, crenated on the margin, and occasionally lobed, which gives them a fan shape. The fertile frond is longer than the barren one, and ends in a panicle or thyrse. The sporangia, which are large and bivalved, are disposed in two regular series upon the divisions of the panicle, and directed towards the upper or inner face of the frond. This fructification bears a strong resemblance to that of Osmunda regalis, the Flowering fern.
The stem of this plant has been assumed to be solid and branched, which is by no means the case; for, upon dissection, it is found to be hollow, and at its base the fronds of the following year may be detected more or less perfectly formed, and the rudimentary bud of the succeeding year within the latter; the position of the barren and fertile fronds being reversed in the successive developments.
SECTION IX.
EQUISETACEÆ, OR HORSETAILS.
The Equiseta are leafless, herbaceous plants, annually renewed from a creeping rhizome, and growing in marshy land, in pools and ditches, on the banks of rivulets, and in rivers, from Lapland and Siberia to within the tropics. There is but one genus, and few species. The largest of the ten or eleven species, which are indigenous in Great Britain, is not more than five or six feet high, but they are of greater size in warm climates. The Horsetails begin their lives precisely like the Ferns; for, when a spore begins to germinate, it forms a marchantioid leaf or prothallus lying flat on the ground, upon which are produced antheridia, full of cells, in each of which there is a spermatozoid with numerous cilia. Archegonia are also formed on the prothallus. These, after fertilization, give rise to the perfect plant, which throws out a rhizome, whence new shoots are produced. ‘The structure of the rhizome is very different from that of ferns. In an early stage it consists of a central column of cellular tissue, sending off about eight radiating plates, which connect it with an external cylinder of the same tissue, and opposite to each of which there is, in the central column, a vascular bundle consisting of annular vessels passing into spiral. At a later period, tissue grows from the walls into the cavities, in such wise that they are more or less perfectly obliterated.’[[75]] From the rhizome, which often extends to a great length, the stems of the Equisetums rise in the form of rough, rigid, hollow cylinders, striated longitudinally, and articulated at intervals by separable joints. Each articulation is invested at its base by a toothed membranaceous sheath, from beneath which, in the greater number of the species, whorls of branches spring, jointed like the stem, and similar to it even to the number of teeth in the sheaths and striæ on the surface, but unbranched.
Fig. 70. Equisetum giganteum:—a, fragment of stem with branches; b, cone or spike of fructification; c, one of the scales of the cone; d, spore with its elastic filaments.
The fructification is occasionally on separate stems, which make their appearance before the barren ones, and are in general unbranched and succulent, bearing a cone on their apex. The surface of the cone is at first smooth or indistinctly reticulated, but it eventually splits into numerous octagonal, brown, shield-like discs, spirally arranged, which, separated from the stem, are found to have a stalk, and to bear on the under-side four or eight pendent sac-like bodies of a whitish hue. These are the sporangia, which open on the inside by a slit for the discharge of the spores. The inner coat of the cells of the sporangium is composed of beautifully spiral tissue. The spores arise by cell division, each being covered by a separate membrane, which ultimately forms a pair of elastic fibres attached by their middle, closely coiled round the spore, as it is formed within the cell; but, when the spore is liberated, they extend; the least moisture, however, even the breath, makes them contract.
The Equisetaceæ differ from the Ferns in having spiral instead of scalariform vessels in their structure; but they agree in having only one period of fertilization, after which they produce a crop of spores year after year, and these, when sown, give rise to the primary leaf with its antheridia and archegonia—a perpetual cycle.