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 be possible to give more than an outline of the reproductive schemes to be observed in a few typical species. These life histories must not be taken as necessarily applying to all the related plants. None the less, by a careful study of the species described we may receive a fair conception of the habits of the class to which it belongs. Incidentally it may be mentioned that even a low-power microscope will be an enormous help in studying the life histories of the Vascular Cryptogams; but if this is not possible, a pocket-lens will help to a better understanding of many of the points described.
For the study of the life history of a Fern one cannot do better than take the commonest of our native species, the Male Fern (Nephrodium filix-mas). Seeing that the general aspects of the plant are fully described in a later chapter, there is no need to enter into such matters at the present moment. We may, however, examine a fertile leaf of the Fern in order that we may start at the beginning of a really interesting romance. A very small magnification of the brown patches on the back of the frond, which we remember are called sori, will reveal their true character. After removing the kidney-shaped cover (indusium) we shall be able to see the spore cases or sporangia quite clearly. Each of these consists of a capsule borne on the end of a stalk. These sporangia are seen to grow out from the sides of a mass of special tissue, known as the placenta, from which the indusium really arises. On occasions a curious club-shaped hair which secretes resin can be observed on the stalks of the sporangia. There seems to be no satisfactory explanation as to the part which this process plays. The capsule of the sporangium is much flattened, and has not been inaptly compared to a watch-case. Its wall is very thin, being composed of a single layer of cells. Around the edges of the little case there is a row of large and thickened cells which form the ring or annulus. Here it may be mentioned again that the structure of this annulus varies greatly in the different families, and is often a useful distinguishing feature. To return to our Male Fern, the annulus is plainly seen to start from the stalk of the sporangium at one side of the capsule, and it can be traced right over the top to a situation about half-way down on the other side. The chief business of the annulus is to bring about the opening of the sporangium in such a way that the spores are violently expelled. This happens in the following manner. When the contents of the sporangium are mature the wall of the capsule, and especially the cells forming the annulus, begin to lose water. The sides of the capsule start to draw inwards, and ultimately the annulus suddenly straightens out and the sporangium is torn open, the actual rupture taking place just at the base of the ring.
The manner in which the spores originate in the sporangium calls for comment. In the case of the Male Fern these arise owing to the repeated division of a single cell. At a certain stage in the process there are produced what are known as mother cells. Ultimately these divide twice, and the resulting cells represent the spores. When ripe, the spores become kidney-shaped and the wall of the cell takes on a rich brown colour. In the different kinds of Ferns, the form of the spore and the sculpturing of its walls vary a great deal. Thus the spores may be globular, oval, or angular in shape; whilst the exterior may be quite plain, or, perhaps, most beautifully marked. The number of spores produced in the sporangium of a Male Fern is usually some forty-eight to sixty-four, although in other species there might be less than the lower figure or more than the higher. To the naked eye the spores appear to be so much dust, and as they are comparatively light they float away on the breezes, and often enough travel for a considerable distance before coming to rest. As a rule the bursting of the sporangia takes place during dry weather. There is a real advantage in this, for when the spores are damp they hang together in masses and in such a state a wide dispersal would be out of the question.