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.
Having studied Ferns in their incipient stages, and also in their past relations, we may now proceed to some practical considerations regarding culture and propagation. To the selective cultivator, and also to the commercial raiser of Ferns on a large scale, the raising from spores is practically the main mode. To the selective cultivator, for the reason that when sowing varietal forms there is always a chance of improved seedlings, or, if sowing diverse forms together, crosses of value may result, neither of which advantages attaches to propagation by division or bulbil, while the wholesale raiser obtains thereby a practically illimitable crop only confined as regards numbers to the demand. In the trade quick-growing exotic Ferns are so rapidly raised in warm houses that the precautions necessary in the case of slow-growing hardy Ferns grown under cold conditions are usually ignored, sowings on non-sterilized soil topped with crushed flower pots being made on a large scale, so that a percentage of failures matters nothing. To the amateur, however, dealing with less favourable conditions, we recommend the following procedure. The spores are best sown as soon as ripe, say in July, not because they do not retain their vitality for years, but because they then germinate almost at once and obtain the full benefit of the growing season. If collected the previous season, sowing in March is advisable for the same reason. Sowings in late autumn are very apt to fail. Having obtained the spores, we take small pots or shallow pans, well drain them, and fill them nearly to the brim with good fern compost - 2 parts loam, 2 parts leaf mould, and 1 part silver sand - rubbed fine and well pressed down, a few crumbs of loam being sprinkled on top. We then place a small piece of paper on the top, and upon this we pour boiling water until it runs out scalding hot at the bottom. This kills all worms, eggs, or germs, and gives the spores a fair field, which from their minuteness is very essential. Covering the pot with a piece of glass, we allow it to cool. The spores, even from a few heaps, will under a microscope prove to number hundreds or even thousands, and it is well to get this fact into one's mind, so that the sowing may be judiciously thin and somewhat in accordance with the sower's subsequent accommodation for the resulting plants. Naturally, too, a thin sowing, giving room from the first for the prothalli, is better than what may practically be equal to sowing a hundred Sweet Peas in a wineglass. The pots being quite cool, the spores should be evenly distributed by gentle tapping of the paper, and the glass cover at once replaced. This replacement, it may be explained, is necessary, as other fern spores or inimical germs may be floating about, and it is well to exclude such as far as possible. The pots or pans may then be put in a shady, cool, but well-lighted place or Wardian case, and if they are small it is a good plan to embed as many as a large pan will hold in fresh coconut fibre, which, if kept moist, will ensure proper conditions in the embedded pots. These should never be watered overhead, but only in case of need by absorption from below. In this case a single pane of glass suffices. In a few weeks, according to the season and the species concerned, the little green prothalli above described will be seen first as glistening green specks, and eventually as full-sized heart-shaped scales perhaps ¼ in. across. These, if, due to too thick sowing, they are greatly crowded, may be safely pricked out in pill-sized patches, 1 in. apart, into larger pans, the soil being preferably again sterilized, though this is not so essential as in the first instance. In a few weeks more, if proper close conditions have been afforded, the first fronds (fig. 307) will be pushing up here, there, and yonder, after which it is only a question of pricking out and bringing on to adult size. If the prothalli hang fire in this respect, a gentle spraying with warm water, or immersion of the pan in such until the water percolating from below just floods the prothalli, may bring about the desired alliances usually effected by means of a dewdrop, as above explained.
Where hybridization is desired to be effected, the two sorts of spores should be sown together on somewhat thicker lines, since cross fertilization can only take place naturally if the prothalli are crowded together. It is, however, beyond doubt that a flooding from below, as mentioned above, when the prothalli are about mature, may have the effect of transporting the fertilizing antherozoids from one prothallus to another and thus effecting a cross. Spores, however, as we have suggested above, do not all germinate with the same rapidity, and hence a sowing of a slow-growing species must be made at one time, and that of its more rapidly developing companion later on, to permit of simultaneous maturity. Some crosses are said to have been effected by severing prothalli of two kinds and bringing the two different halves in close contact, an idea which is quite feasible, as the prothallus is very tenacious of life, and will bear such an operation with impunity.
One point must be remembered in crossing or hybridizing, and that is, that it is practically impossible to determine what is a cross or a hybrid except by the clear appearance of the two parental characters in the offspring. The pollen equivalent cannot, like pollen itself, be collected and applied in the right quarter as with flowers; the material is too minute to handle and the chance of self-fertilization cannot be eliminated, besides all which, in sowing varietal forms, and it is only by sowing two very distinct types that any recognizable result is possible, there is always a great possibility of a purely independent "sport" occurring amongst the seedlings of either form, which may not owe its origin to cross influence at all.
We may now conclude by a few remarks on propagation by other means than spores. The usual methods of layering bulbils of proliferous fronds, or stolons of stoloniferous ones, are too well known to require description. Plants with creeping rhizomes, like the Polypodies, Davallias, etc, only require portions of such rhizomes or fleshy rootstocks to be severed with a frond or two and a growing point to be treated as rooted plants, and plants producing offsets laterally only require these to be prised off and planted for them to establish themselves at once.