Those that have surface rhizomes such as the davallias and some of the adiantums (Capillus-Veneris is one), are easily and quickly divided. A rhizome that has run out and thrown up a few fronds will have also made some roots and can be severed from the parent plant and potted. Never overpot ferns. It is true that some of the stronger growing kinds, especially the pteris, soon get root-bound, and then want larger pots, but the great majority of ferns do not need so much pot room, but they always want good drainage, so that water can pass freely through.
Those that have stolons or under -ground rhizomes, such as the nephrolepis, are most easy of- all to multiply, young plants often coming up at the side of the pot and on the aerial roots, the young plants appearing at intervals. In June, when your bedding plants are gone, select a bench that will let the water through freely, and in four or five inches of soil plant out young plants from 2-inch or 3-inch pots. They will by September or October have made fine plants, and have sent out such an abundance of stolons that at intervals, or when you lift, you will get a number of young plants, which can be potted up, or if a larger stock is needed, replanted. There is no doubt a much larger plant of any of the nephrolepis can be obtained in a short time by planting out than if grown in a pot, and they lift with a mass of roots perfectly, without losing a single frond. Yet specialists growing these nephrolepis into medium sized specimens prefer growing them entirely in pots, at least from a 3-inch or 4-inch size.
Few of the commercial sorts are proliferous on the leafy frond, but those that are lend themselves to propagation most easily, as described in the remarks on that class.
Some species that grow in tufts, such as Adiantum cuneatum, the common maidenhair, can be divided. The crown should be cut carefully and then the roots pulled apart. Cuneatum, or any particular form of it, is often increased by division, and sterile species, of which the most beautiful of all, A. Farleyense, is one, can only be propagated by division. This should be done in early spring, when the plants are in most cases resting and before the young growth is made, but can by care be done at any season.
Just here it is worthy of mention that this beautiful fern, A. Farleyense, is usually thought to be a sterile form of A. tenerum, but there is no definite knowledge about it, and the millions of plants now existing, or that have existed, all came from one plant found growing on Farley Hills, in the Island of Barbadoes, the thickest populated island of the world, where the children's stomachs are distended like balloons by an unchanged diet of sugar cane.
Nearly all the useful species can be readily raised from spores, which is the natural way, and has the advantage of producing possibly either an improved form or variation from the parent which by division, or by proliferous stolons, or divisions of the rhizomes, never happens. The raising of seedling ferns from spores is a very delicate operation and with the beginner not always a success. You will likely get several species which you never believed you sowed and few of those that you thought were sown. We all know how ferns spring up in the pots or on the bench if left undisturbed for a few months, if there are any spore-bearing ferns in the house. Adiantum cuneatum I have seen vegetate on a slimy, dirty brick wall by the tens of thousands, and had to scrape them off for the sake of cleanliness.
Before giving any directions for sowing, just a word about these spores. The whole order of ferns has no flowers, consequently no sexual organs, and from the spore to the young perfect fern frond like its parent is a profound, complicated and mysterious phenomenon. When the spore vegetates it forms cells, which are called the prothallus, and is only an increase of cells. On the under side of the prothallus (which resembles the liverwort so often seen on the surface of our soil with plants making a slow or stagnant growth) develop the organs of both sexes, which have the same function as the more conspicuous organs in the flowering plants. To describe the complicated and marvelous process of fertilization would require a chapter, had I time or space to quote it. From the prothallus finally springs (varying in time with the species) the young true frond. Any cross fertilization of species, as we do with flowering plants, is therefore impossible, but by sowing the spores of different species in one pan there is assurance that hybrids have been produced, and this is getting deep into science.
One word as to the fertility or fecundity of ferns. Mr. Charles T. Druery, who is quoted in the "Book of Choice Ferns," says: "We have estimated the spores upon a single frond of our native (British) Polypodium vulgare (a frond not over a foot long by three inches wide), and found that one of the subdivisions of the same size taken from a tree fern would yield plants sufficient to form a wood as large as Epping Forest. Every frond would bear hundreds of such subdivisions and the tree fern would probably bear thirty or forty fronds every season. A little calculation, therefore, will show that inconceivable numbers have to be dealt with." Truly inconceivable; countless millions on every frond. Another illustration by the same author was the shaking of the spores of an asplenium out and collecting them, about filling a teaspoon in which he estimates he had eighty million spores. So if one in 10,000 of the spores we sow vegetates, we are doing well, and the surface of our pan will be covered with the moss-like prothalli.
The spores should be gathered, or rather the frond cut before the spore cases have burst, and if not convenient to sow at once, put them away in paper bags. The soil or material you sow on, which can be a light, sandy loam, covering an inch or so of broken crocks, should be baked to destroy all germs of weeds or moss or eggs of insects. If not baked, water with scalding water. Make the surface smooth and scatter the spores. No careless watering must be given, but let it flow over the surface slowly.
If covered with glass, which it should be, the soil will not need much watering till the prothalli appear in the way above described. When this about covers the surface of the pan they should be divided by taking small patches, say a quarter of an inch square, and placing them on the surface of other pans or flats. Soon the true fern leaf will appear, when in time the little plants can be pricked out singly in small pots, or, what is still better, in flats, till they are larger and need a pot for themselves.
If a few large plants of the leading sorts are kept in a house and allowed to shed their spores, they will be carried to every corner of the house, and if some plants (like large palms) are in the house that are not often shifted, you will be sure to have an abundance of young ferns. I have noticed frequently the young plants of Adiantum cuneatum growing on the sphagnum in the cattleya baskets.