This section is from the book "Commercial Gardening Vol2", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
The life-cycle of flowering plants, which may be shortly stated as seed, plant, flower, and seed again, is simple enough to be popularly understood; but that of the Ferns is so much more complicated, and evidenced on such a microscopic scale, as to be mainly unknown even to those who grow Ferns or appreciate their beauty. In the first place, the Fern is differentiated from the flowering plant by bearing neither obvious flowers nor obvious seeds, although, as we shall presently see, microscopic equivalents of these are really produced in the course of the events which precede the appearance of a second generation of young Ferns. Instead of a seed proper, which is always the result of fertilization brought about by the conjunction of reproductive cells existing in the pollen grain as the male, and in the embryo seed as the female, the Fern produces on its fronds an enormous number of asexual spores, microscopically minute bodies which consist only of a reproductive cell protected by a husk or shell. These spores are contained in tiny capsules or pods (sporangia) (fig. 303), and their minuteness may be conceived when it is stated that even these pods are only definable as such under a good lens, though they contain, as a rule, several scores of spores apiece. These capsules in their turn are collected into groups (fig. 304), and it is the method of these groupings and arrangements generally which has enabled the botanists to classify and divide Ferns into a large number of distinct genera, determined by such arrangements, while these genera are again divided into species, which all have the same form of fructification, but vary in make, habit, and other characters on more or-less definite lines. In the vast majority of cases the spores are arranged on the backs of the fronds in isolated dots or roundish heaps (Polypodium) (fig. 305 a), in definite lines along the veins within the margin (Asplenium) (fig. 305 d), in continuous or interrupted lines along the margin (Pteris, fig. 305 E, Scolopendrium, Adiantum, fig. 305 f), in continuous even layers covering the surface (Platycerium); while in some genera, such as Osmunda, Anemia, and others, the fructification is confined to definite portions of the frond, or as in Struthiopteris and Onoclea to entirely specialized fronds, a more or less clustered form being adopted at the expense of the leafy development. Apart from these and many other types of arrangement, characterizing different genera, the groups of sporangia are differentiated by the presence or absence of a protecting cover called the indnsium (fig. 304 i), and the particular form which this cover assumes. Thus we have three native genera both characterized by bearing the sori or spore heaps in a round isolated form, viz. Polypodium, Polystichum, and Nephro-dium (Lastrea). The Polypodium, of which we may take one common species, P. vulgare, or the exotic P. glaucum as types, has these heaps entirely destitute of a cover, as, of course, have all its generic relatives. The other two, on the other hand, have distinct semi-transparent scalelike coverings or indusia, but are differentiated by those of Polystichum being quite round and attached to the centre of the heap by an almost imperceptible stalk, while in Nephrodium, although similarly attached, the cover is definitely kidney-shaped, the attachment being at the deep indentation. Blechnum and Lomaria, two very similar genera, bearing the spores on upright contracted fronds springing from a rosette of leafy ones, are often confused even by the botanists who have determined the difference, which is that in Lomaria the lines of spore heaps are purely marginal, with the margin turned backward to act as indusium, while in Blechnum the heaps or lines are only near the margin, which is unaltered, a distinct and independent indusium arising between it and the spore heaps. From what we have stated, it is clear that a little study devoted to the forms in which the spores are borne is of great assistance in determining to what species the plant concerned belongs, apart from the general appearance, which may mislead.
Fig. 303. - Sporangium of Cystopteris.
a, Annulus; b, broader cells of same where bursting takes place (magnified).
Fig. 304. - Section of Leaf and Sorus of Nephrodium Filix-mas 8, Sporangia; i, indusium (magnified).
Fig. 305. - Showing Pinnules and Sori of various genera of Ferns.
A, Polypodium. B, Aspidiura. C, Nephrodium. D, Asplenium. E, Pteris. F, Adiantum. G, Sorus of Hymenophyllum.
Having thus dealt with the spore, or rather the spore heap, as a discriminating feature for Fern identification, we may now go a step further and consider how this unfertilized one-celled simple body can effect its task of engendering a new generation, involving as this does something equivalent to flowers and the two sexual interacting elements which flowers produce prior to the evolution of a fertilized seed capable of yielding a young plant. A seed, as we know, enjoys the advantage of containing not merely a fertilized germ but also a store of nourishment with which the mother plant had endowed it after the fertilization was effected in order to give it a start in life. The spore possesses no such advantage; it is only endowed with sufficient vitality to swell under congenial conditions of warmth and moisture, and, bursting the husk, to protrude a self-engendered root-like cell which attaches itself to the soil; and by virtue of the nourishment obtained therefrom and from the air through the chlorophyll, which even at this early stage it contains, it slowly forms, first a short chain of cells (fig. 306) and then by lateral multiplication as well, it builds up a heart-shaped dark-green growth (fig. 307) about the size of a herring scale, which is firmly rooted to the soil by innumerable root hairs or rhizoids, mainly springing from the first-formed part of the "prothallus", as this body is termed. Viewed from above, the prothallus looks simple enough, and resembles anything but a Fern, a fact which rendered the origin of young Ferns a profound mystery until so recent a date as 1846, when Count Suminski discovered the final stage in the normal life-history of the Fern, and the mystery was one no longer. Detaching one of these scales from the soil, reversing it, and studying it under a good lens, we find it to be a very wonderful structure indeed. Dotted about profusely among the tuft of root hairs we see a number of roundish pimple-like growths (fig. 308 an): and farther up, close to the indentation of the heart-shaped prothallus, we shall see a little close-set cluster of teat-like bodies (fig. 308 ar). Beneath these we may see that the body of the prothallus is considerably thickened, and dissection would show that at the base of each of these teat-like bodies a round incipient seed was embedded. Assuming, then, as we shall be justified in doing, that these teat-like bodies represent the stigmas in flowering plants, and deducing therefrom that the round pimples should represent the male or pollen element, we see at once that the simple one-celled spore shed from the parent fern has managed to construct a very complete apparatus for the performance of the reproductive function. The only remaining question is: How is it done? If, while the prothallus is so reversed, we flood it with tepid water, we may, if we are very fortunate, see some of the round pimple-like bodies (antheridia) burst, and from them will be sent a considerable number of extremely minute ciliated bodies (fig. 309b) (antherozoids), which commence to swim actively about in the water (normally existing in the form of a dewdrop), actually steering themselves definitely towards the teat-like bodies (archegonia) the special cells or oospores at the bases of which it is their province to fertilize, since each contains, as does a pollen grain, the male potency essential thereto. Arrived at the archegonia, they find their way to the incipient seeds (oospores) at the base of the hollow they have traversed, and from that moment the fern "seed" begins to develop by cellular multiplication. Ere long a little frond rises through the indented parts of the heart-shaped prothallus (fig. 307), a little root (r) descends, the fleshy prothallus acting for a time as foster mother, and in this way the young Fern is fairly launched in life after, as we have seen, a very wonderful pre-natal career.