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.
Fig. 306. - A Young Prothallus arising from a Spore.
p, Prothallium; rh, rhizoid; r, root.
Fig. 308. - Underside of Prothallus of Fern.
ap, Apex; r, rhizoids; an antheridia; ar, archegonia (magnified).
Fig. 309. - A, Archegonium ejecting mucilage (m); o, oosphere. B, Antheridium ejecting antherozoids. pr, Prothallium (highly magnified).
The life-history of a Fern may thus be summarized. Fern, spore, prothallus, antheridia, archegonia, incipient seed, Fern again. This, however, is the normal course; but Nature in her infinite variety has contrived to bring about the same results - i.e. Fern from Fern - by numerous short cuts. Thus in the familiar Ferns, Asplenium proliferum and Nephrolepis exaltata, we see the shortest cuts of all, since the parent Fern bears in the first case a veritable multitude of youngsters on its fronds, youngsters so precocious that they often bear another generation of their own before being detached; and, in the second case, by a multitude of stoloniferous runners on Strawberry lines, by which it can be propagated ad libitum. These represent the types of numerous Ferns of bulbiferous or stoloniferous habit in which a new generation is produced by direct outgrowth of the parent, and to this class belong those Ferns which produce lateral offsets. These proliferous forms, however, relate to the adult Fern, and do not affect the pro-thallic life-cycle already described, and which has also been varied in every conceivable way. Some years subsequent to Suminski's elucidation of the normal cycle, Professor Farlow, in the United States, demonstrated that with some Ferns (amongst them our Lastrea pseudo-mas cristata) a nonsexual bud was generated on the prothallus and produced the parental type, the antheridia and archegonia and incipient seed being thus eliminated, an ordinary bud being produced instead. The writer then discovered on a form of Lady Fern (Athyrium filix-foemina, var. clarissima, Jones) that certain excrescences yearly produced on its frond backs, and which had persistently refused to yield plants by means of the sporeheaps which they were assumed to consist of, were really capable, when layered, of developing prothalli direct without the intervention of the spore, these prothalli proving to be perfect by the production of numerous characteristic plants. This implied the elimination of the spore and spore cases, which Professor Bower eventually found were partially aborted at an early stage, the prothalli eventually growing from the remaining cells. This was termed "apospory", or spore elision. Mr. G. B. Wollaston immediately followed with an example of a Polystichum (P. angulare, var. pulcherrimum), in which the extreme tips of the subdivisions of the fronds grew out into prothalli which produced plants, though of imperfect character. This cuts out the sporeheap altogether, and has been termed "apical apospory" and the first found form "soral apospory" to differentiate them. Since then this phenomenon has been found in several other species, mostly of British Ferns and in varietal forms. Dr. Lang's experiments with spores of abnormal varieties went even the length of demonstrating that a Fern might possibly be perpetuated indefinitely without getting beyond the "prothallium" stage, since he found spores produced on the prothallus itself; and if these had produced offspring (which they failed to do), and inherited the retrogressive character, this would have constituted a reversion to a very low type of vegetation indeed, resembling the Marchantias or Liverworts.
To the Fern-cultivator a good grip of the particulars we have given of the career of the embryo Fern will be of value when we come to consider a little later on their propagation by the several means at command. Meanwhile, however, as part and parcel of the life-history of the Fern, we think it well to say a few words on those of the past. In the first place, the Ferns, as spore bearers, belong to that great family of plants, similarly characterized, which in the far distant Carboniferous Age formed those forests of immense area which in their fossil state now represent the coal seams upon which so vital a portion of Britain's - and indeed the world's - prosperity is founded. The mind utterly fails to conceive the immensity of time which lies between that period or periods and the present time. Thousands of feet of superincumbent formations have been piled upon them, each one a page in the history of the world, involving perhaps changes from high land to deep ocean, and, in any case, immensely long periods for their deposition, to say nothing of the intervening pages which have been deleted by erosion. Nevertheless, it is a very remarkable fact that the Ferns of even that far-distant date strongly resemble those of the present, though it is presumably a fact that in the same period practically all the wonderful diversity of fruit and flower has been evolved from plants which at that time were really Ferns or their allies, or had only commenced to be evolved with the true floral character. One fact which had probably contributed to this practical standstill in Fern development is the microscopical character of their inflorescence, which has prevented them from benefiting by that interaction of the insect world which has played so powerful a r61e in floral evolution. In this connection it is a curious fact that all through the Ferns, whether assuming the size of a magnificent Palm, as in Dicksonia, Cyathea, etc, or that of a minute grass tuft, as Asplenium septentrionale, not only are the spores equally microscopic, but the prothallus is practically indistinguishable, and the fertilization nominally is always self-effected, i.e. by the adjacent antherozoids of the same prothallus, a fact which throughout nature tends rather to degradation than upward evolution. Since Ferns have undoubtedly sprung from the seaweeds which formed the antecedent types of primary vegetation, and yet, as Ferns and allied plants, were so definitely developed in the Coal period, we are necessarily confronted with another inconceivably long period of time precedent even to that; a consideration, however, with which we need not burden our brains, as it is utterly beyond calculation.