Propagating Fern Spores
The propagation of ferns by simple division has been described fully in the section on cultivation, and does not need any further elaboration here. It is the simplest method to adopt when it is not important to build up a large stock, and some ferns multiply their crowns fairly rapidly, such ferns as the Brittle Bladder Fern, Cystopteris fragilis, and some of the Lady Ferns, Athyrium f. f. minutissimum for instance, soon making a clump of small crowns which can be separated and grown on without trouble. However, it is the only means of propagation which can be used in the case of certain ferns which normally are sterile, never or very seldom forming viable . The beautiful wild sport of the Hard Shield Fern, aculeatum pulcherrimum ‘Bevis’, is an example, though fortunately this fern is fairly liberal in making offsets.
Some of the varieties of Lady Fern, or cultivars as I suppose they should be called will come fairly true from. The Tatting Fern, A. f. Frizelliae, for instance, comes almost a hundred per cent true, but others such as A. f. f. coronatum and A .f .f. gemmatum come anything but true, say five per cent or less, the other seedlings being often well worth growing, but not the true variety. In this case division is the only method to use in maintaining the true stock.
As propagation by division is a slow process, it may take many years to build up stocks adequate for commercial distribution, so plants which cannot be increased by quicker methods will always have a high market value.
The production of buds on the frond bases has been mentioned in an earlier section, in the case of the Male Fern, Dryopteris filix-mas, and this occurs to a greater degree in the Soft Shield Fern,setiferum and its varieties, to a lesser degree in other species, and this property enables one to increase them at a more rapid rate.
Sometimes when the growing point of Dryopteris or Polystichum has failed, for some reason or another, and the plant appears to be dead, the rootstock and frond bases still may have life in them. If all the dead material is removed carefully with a sharp knife until only plump, still green material is left, this should be well washed to remove all traces ofand placed in sterilized sand in clean pots and kept very close. I find inverted jam jars quite useful for this. If there is life enough left in the material, bulbils or buds may be induced to grow, ultimately making small plants which can be detached and grown on, thus saving a possibly rare variety and increasing it considerably.
A special case of bulbil production is found in the Hartstongue, Phyllitis scolopendrium, and its many varieties. Here the old frond bases remain green and plump long after the fronds have withered and gone, two or three years perhaps. If the plant is lifted, all soil washed away, and the oldest moribund part of the rootstock cut back until live tissue appears, the frond bases can be snapped off or cut away with a sharp knife, as near the rootstock as possible, up to within an inch or so of the growing point. The top portion with fronds can be replanted in good soil when it will grow away again, making new root and all the better for having the old parts removed.
The bases are washed, trimmed free of bits of root and any brown dead matter, and then sown over the surface of clean washed sand in a clean seedpan or other container. Covered with clean glass or polythene (I must emphasize the necessity for cleanliness, hence the repetitions) and kept close, out of direct sunlight, after some weeks they will produce tiny white bulbils, and anything from one to a dozen tiny plants may develop from each base. These are removed when large enough, pricked off into sandy fern compost and kept close until growing freely, when air can be admitted gradually until the plants are hardened enough to have the glass removed altogether. The young plants can then be potted up in very small pots or lined out in nursery beds. This is the best method for getting a good stock of the barren Phyllitis s. crispum in its many forms, division being a very slow method. Of course, the young plants, being parts of the original, will be true to variety.
In the case of the Soft Shield Fern, Polystichum setiferum, especially in the acutilobe and divisilobe sections, bulbils are freely produced along the rachis (midrib) at the base of the pinnae; these are evident as small scaly knobs. To induce these to grow into plants the frond may be detached and pegged down on to gritty leafmould in a box, and kept close in a propagating-case or frame in the shady part of a greenhouse. Before very long roots will form and young fronds appear. When large enough to handle the young plants should be detached and pricked off into boxes of fern compost and grown on until large enough to line out in a shady border.
If there are but few bulbils near the base of the frond only, it is be ter to layer the frond in situ, still attached to the plant, by placing gritty leafmould under the frond and pegging it down in close contact with the soil. When the young plants are growing freely they should be detached carefully and lined out as before, leaving the old frond still attached to the plant.
Some forms of this fern are slow to produce bulbils, and to induce them to do so liberal topdressings of leafmould with a generous admixture of bonemeal placed round the parent plants should be tried. A well-grown, flourishing plant is more generous in forming bulbils than a starved one.
When large numbers of plants are required the best method is to sow spores, especially in the case of true species and varieties which are known to breed reasonably true. It is also the best way to obtain improved forms of varieties if there is room enough to grow on a large number of young plants for two or more years, with a view to selecting the best ones and getting rid of inferior forms. Only perfectly symmetrical parents should be chosen as a source for spores, as there is a tendency for irregularities and deformities, such as constantly irregular pinnae and tendencies to revert, to be transmitted to the offspring.
When one has grown on large batches of sporelings for two or three years, their true character should be more or less apparent, and any which are not up to standard, or an improvement on the parents, should be discarded ruthlessly. If one is not rigorous in this respect, the whole garden will become full of rough, uneven, rampageous plants competing with and perhaps overpowering their betters. It is really difficult to harden one’s heart to get rid of the poor ones, often so much trouble, but one is repaid by the excellence of the chosen few. The discarded plants need not be altogether wasted, as their roots, washed clean, can be used in orchid compost. Or some friend or local nurseryman might be glad to have the fibre for use in making up composts.
The technique of spore raising varies among growers, but the basis of all methods is to give the spores a chance to grow into prothalli before being invaded and swamped by mosses, fungi, insects and other horrors. In nature only a tiny percentage of the fern spores shed succeed in reaching maturity, which is just as well, as already pointed out, considering the billions upon billions of spores shed annually
Therefore all materials and containers must be sterilized before use. Personally I use an old method which serves me well enough. Suitably sized earthenware seedpans are well scalded and drained dry. A piece of perforated zinc is placed over theholes to exclude the entry of worms, which can cause havoc. Then some fern compost, equal parts of loam, leafmould and coarse sand, is well mixed and sieved through a quarter-inch mesh. The coarser part left in the sieve is placed in the bottom of the pan an inch or so thick, and then topped up with half an inch or so of the fine-sieved soil.
Pressing firm and level — I use the bottom of the next pot — the surface is protected with a piece of paper, and a kettleful of boiling water is poured on to the paper until the pan and contents are too hot to hold. When cool enough the paper is removed and the soil protected from contamination by covering the pan with a sheet of perfectly clean glass. When quite cold, a tiny heap of spores is picked up on the point of a knife and shaken evenly over the soil, the pan is labelled, covered at once, and placed in a shady greenhouse or cold frame or even on a shady window-sill. The glass should not be removed; when watering is necessary, the pan should be stood in a saucer of clean water deep enough to reach the bottom of the compost. Any condensation under the glass cover helps to maintain a moist atmosphere and should not be removed. Every time the cover is removed there is a risk of injurious organisms obtaining access.
It may be anything from one to six months before a green film appears on the surface of the compost, and several more weeks before the scale-like prothalli can be seen. After a few more weeks the first tiny fronds may be seen arising from the prothalli, and when large enough to handle they should be pricked off and kept growing freely being kept close until well developed, when more air may be given; the young plants are hardened off gradually, finally dispensing with the glass cover.
Pot or prick off farther apart when large enough and finally line out in a prepared bed. From sowing to potting may take two years, but if sowings are made season after season the time passes quickly enough. If, as is quite likely — I do it myself — you have sown the spores too thickly, it will be necessary to prick off the prothalli in tiny bunches into pans of sterilized soil, so that they can develop properly. This needs care and good eyesight; the prothalli will stand some handling, but must not be allowed to get dry. If left in a dense mass, most of the prothalli may not develop archegonia at all and very few seedlings will arise.
In this plastic age there are many containers winch could be used instead of earthenware pans. The polystyrene seedpans stand up to a dose of boiling water without distortion. The chief virtue of these pans is the length of time taken to dry out; the preliminary soaking often lasts until germination has taken place. They are of course covered with sheets of glass. In fact on a flat base, one sheet of glass efficiently covers as many pans as can be got under it without air gaps, as their size is uniform, whereas earthenware pans need covering individually as they never are quite uniform in shape and size.
Plastic containers which will not stand up to boiling water can be used if the compost is sterilized and kept from contamination in a separate container, then placed in the sowing container under conditions which ensure that stray spores do not fall on it. Failure to achieve freedom from stray spores generally results in a strong growth of moss which effectively strangles the prothalli.
I tried sowing in conical glass flasks, as used in raising orchids, plugging the necks with cotton wool and covering this with polythene. The flasks with soil within were sterilized previously in a large pressure cooker — the poor man’s autoclave, but equally efficient — and of course cooled down before sowing. Many greenhouse ferns were very successfully raised by this method, but, rather oddly, the hardy fern spores, sown at the same time, did not do at all well. Out of two hundred flasks sown, perhaps a dozen or so did well. The amount of moisture present might be a critical factor, but I did not do any research with the method.
Instead of glass covers, to avoid loss of moisture and the need for watering, I tried putting the sown pans in clean new polythene bags with a cane inserted in the pan to act as a tent-pole, holding the polythene in the form of a tent. New canes were used, but they should have been sterilized before use, as in many cases a circle of black mould spread from the canes and eventually covered the pans, so, no prothalli. The pans which did not get infected did well.
An excellent method of sowing spores, is to grow them in clean sterilized pots stood in saucers of dilute potassium permanganate, and covered with glass. Another method is to scald earthenware plant pots; and then when cold, stuff them with sphagnum moss, invert them in a saucer of sterilized water and cover with a belljar, or put in a small frame which has been sterilized. The spores are sown on the surface of the inverted pot. This method works quite well.
The simplest method is to sterilize a patch of the greenhouse earth floor by pouring boiling water over the area. Protect the area with a clean slate until cold, sow, and cover with a glass belljar.
The slate prevents contamination with unwanted spores until the ground is cool enough to sow.
In all cases sowings should be labelled as soon as made, and a record of all sowings kept in a notebook, showing source of spores, date sown, date of germination, and date when pricked off, with a further space left for remarks on results. Such a record will be useful for future reference and for checking failures.
To obtain the spores, a frond or part of frond of the desired fern should be examined with a strong lens to find out the condition of the sporangia, from midsummer onwards. When they appear to be ripe and before they have opened, a portion of frond bearing sori should be taken and laid down on a sheet of perfectly clean white paper. I use white sulphite flower-wrapping paper of such a size that the paper can be folded over the frond to protect it from dust, and kept in a dry place where it will not be disturbed. After two or three days an impalpable powder-the spores — will be shed, and these should be tapped very carefully on to a small square of clean paper, neatly folded and kept in a clean new seed packet until convenient to sow. Except in the case of the Royal Fern, Osmunda, and its near relations, whose spores remain viable for two or three days only and must be sown as soon as they are shed, most fern spores remain viable for years; but the sooner they are sown the better the results.