Although they vary somewhat in size, all the Horsetails are striking plants. Here there is a branching underground rhizome from which arise the aerial stems. The most distinctive feature of the plant are the whorls of smaller branches which arise from the joints of the main stem. These carry on the work which is usually assigned to the foliage of the average plant,—that is, the assimilating of carbon from the carbonic acid of the atmosphere. The real leaves of the Horsetail are much reduced in size, and take very little part in the work of nutrition. We shall find them at the joints of the stem as rings, each collection forming a kind of sheath. The leaves, which are usually of the same number as the branches, show no sign of their individuality, save in the little projecting teeth. In some species the fertile shoots, which appear in the form of cones, are produced specially. These appear in the spring before the ordinary vegetative growths, and are quite destitute of chlorophyll. In other species the normal green shoots are fertile at the termination. The sporangia are borne on curious scale-like leaves, a large number of which go to the making of a cone.
One or two aquatic plants, which belong to the Vascular Cryptogams, call for comment. The Pillwort is a singular plant not uncommon in damp situations. The leaves of this plant are narrow, and the spores are produced in curious rounded processes. The Water Fern (Azolla) is an introduced plant which sometimes grows abundantly on lakes in the South of England. Both the before-mentioned plants are allied to the Ferns. The Water Club Mosses (Isoëtes) are represented in this country by a species commonly known as the Quillwort. This plant grows in lakes, and is easily recognized by its quill-like foliage.
Owing to the large number of species a somewhat elaborate classification is necessary in the case of Ferns. In distinguishing the different families, the manner in which the collections of spore cases, known as sori, occur, as well as the features which the individual sporangia present, are important guides. The actual position of the sorus on the leaf, the presence or absence of a covering (indusium), are also distinctive features, both in the families and sub-families. When the individual sporangium is examined it is found that there is often present an annulus, a special ring of cells which plays an important part in the rupturing of the case. The extent of this ring or (as sometimes happens) its absence will alike be a decisive factor in fixing the family to which a species belongs. In some families a prominent feature is the fact that the sporangium has little or no stalk, although this is the exception rather than the rule. For a more complete description of the sporangium of the Fern the reader is referred to a succeeding chapter. It is certainly helpful to a study of these beautiful plants to try to fix in the mind the families, and their characters, of the order Filices. In all there are eight families belonging to the Fern tribe. These are given in the order in which they occur in technical books.
1. Hymenophyllaceæ.—The Filmy and Bristle Ferns. This family includes some of the simplest kinds of Ferns. There are only three representatives in the United Kingdom. These are Hymenophyllum tunbridgense, H. Wilsoni, and Trichomanes radicans. The two former species are fairly common on rocks which are splashed with water, but the latter seems only to occur in restricted districts in the South of Ireland. All the species must have an abundance of water, or the foliage quickly shrivels. This is due to the fact that the leaves consist of a single layer of cells and are, of course, very thin. A distinctive feature in this family is the bearing of the sporangia; these are almost or entirely stalkless. The sorus, as the group of sporangia is called, is surrounded by an enclosure from the leaf margin. In Trichomanes this is cup-shaped, whilst in Hymenophyllum it is bivalved. The popular name Filmy Fern—bestowed on the Hymenophyllums and allied species—has reference to the semi-transparent nature of the fronds. In the case of Trichomanes the axis on which the sporangia are inserted often projects beyond the cup in which they are contained. This gives a curious spiky appearance to the fertile frond, and hence the name Bristle Fern.
2. Polypodiaceæ.—This is a very large family, containing two or three times as many species as all the rest of the Vascular Cryptogams put together. Nearly all our native species, with a few exceptions, belong to the family. A distinctive feature is the incomplete annulus of the sporangium. Another point to notice is that the spore cases are stalked. So large is the family that it has been divided into a number of sub-families; the members of these are chiefly characterized by the position of the sorus, the cluster of sporangia on the back of the frond. The different sub-families may be briefly outlined.
(a) Davalliaceæ.—There are no British representatives of this family. In this case the sorus is always near to the margin of the leaf, and the indusium or covering is cup-shaped. A familiar species is Davallia bullata from the East; the rhizomes of this Fern are trained into various shapes by the Japanese.
(b) Pterideæ.—The Bracken Fern (Pteris aquilina), the Maiden Hair (Adiantum capillus-veneris), and the Parsley Fern (Cryptogramme crispus) belong to this sub-family. A notable feature of the Bracken is the continuous marginal sorus. There is no proper indusium, but the leaf margin curls over and protects the sporangia to some extent.
(c) Aspidieæ.—The sorus is in the form of a little rounded heap. The indusium, which is usually kidney-shaped, is supported by a central stalk, somewhat after the manner of a nasturtium leaf. The Male Fern (Nephrodium filix-mas) belongs to this sub-family, as well as the Bladder Ferns (Cystopteris) and the Woodsias.
(d) Asplenieæ.—Here the sorus is elongated or linear. The indusium arises from a vein to which the sorus is attached. Some very charming Ferns belong to this sub-family. Many botanists include the Lady Fern (Athyrium filix-fæmina) in this section. Certain of the Spleenworts (Asplenium) are common. The Wall Rue (A. ruta-muraria) and the Black Maidenhair Spleenwort (A. adiantum-nigrum) are well known.
(e) Polypodieæ.—The sori on the underside of the leaves are without any indusium. They are in rounded clusters, and look like small buttons. Polypodium vulgare is one of our commonest Ferns. Some of the other species of this genus, such as the Oak Fern (P. dryopteris) and the Beech Fern (P. phegopteris), are abundant in some localities.