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.

(f) Grammitideæ.—The Gold and Silver Ferns. The only British species is the Annual Maidenhair (Gymnogramma leptophylla). The plant occurs in the Channel Islands. This species is one of the few Ferns which are not perennial. The sori, which follow the veins, have no indusium.

(g) Acrosticheæ.—There are no British representatives of this sub-family. In this case the whole of the underside of the leaf is covered with sporangia, and there is no indusium.

3. Cyatheaceæ.—There are no British representatives of this family, which is interesting, owing to the fact that it includes the Tree Ferns.

4. Gleicheniaceæ.—A group of Ferns which are almost entirely tropical.

5. Schizæaceæ.—Another tropical family.

6. Marattiaceæ.—A family of large and handsome Ferns, the members of which occur in the tropics. There are not many representatives of this family nowadays, but remains in the Coal Measures show that the species were very much more numerous in Palæozoic times.

7. Osmundaceæ.—A small family, but rather an important one, owing to the fact that a leading representative, the Royal Fern (Osmunda regalis), is so well known. In this species only the upper portion of the leaf is fertile. The sporangia have very short stalks, and are not provided with an annulus at all. They burst open in a longitudinal slit, opposite to a special group of cells just below the apex. The sorus has no indusium.

8. Ophioglosseæ.—This family is represented by three British species, of which the Moonwort (Botrychium lunaria) and the Adder’s Tongue (Ophioglossum vulgatum) are best known. There is much doubt as to whether this family can be properly included amongst the Ferns at all. We may here give them the benefit of the doubt. The leaves in these species are unfolded from the sides—a totally distinct plan from that to be observed in all the Ferns which have been described, where the frond and its divisions are unrolled upwards. The prothallus is a small underground body destitute of chlorophyll. The fertile leaves are distinguished from the barren ones by the production of a special branch which bears the fructification. The sporangia are large.

The next order of the Vascular Cryptogams is of comparatively small importance as far as the present study is concerned. It is known as the Rhizocarpeæ (Pepperworts). The order is divided into two families as follows:—

1. Salviniaceæ.—The only two genera are Salvinia and Azolla; the latter has been already mentioned.

2. Marsiliaceæ.—The British example is the Pillwort (Pilularia globulifera).

The Club Mosses have been divided into six families. Two of these—the Lepidodendraceæ and the Sigillariaceæ—are only represented by fossils; and one, Psilotaceæ, has no British representatives. The remaining families all include one or more species which are indigenous to our islands.

1. Lycopodiaceæ.—These are the Club Mosses proper. Several species of the genus Lycopodium are British. The Common Club Moss (Lycopodium clavatum) is often abundant on high moors.

2. Selaginellaceæ.—A large family containing three or four hundred species, only one of which, however, is British; this is Selaginella spinosa.

3. Isoëtaceæ.—A family of aquatic Club Mosses. The British species is Isoëtes lacustris, a plant which is sometimes common in the northern lakes.

With this brief survey of the Vascular Cryptogams one may naturally pass to a somewhat more detailed consideration of the life histories of these interesting plants than it has been possible to give in an opening chapter.

CHAPTER II
LIFE HISTORIES

Even the most general survey of the Vascular Cryptogams would not be complete without an attempt to indicate the means of reproduction to be observed in these plants. The subject is one which might well be treated at great length, for there is scarcely any species which does not present some interesting point that calls for comment. Within the limits of the present inquiry it will not