VEGETATIVE REPRODUCTION IN FLOWERING PLANTS

The use of stored food in structures, such as corms and tubers, for vegetative reproduction has already been dealt with on pp. 298-305. The ability to spread and multiply is not limited to such specialized organs, but is commonly possessed by all those plants which normally can produce adventitious roots. In plants like the ground ivy and bugle the horizontal shoots arise from axils near the surface of the soil. Wherever the nodes of such shoots come in contact with the soil, adventitious roots arise and the buds at the node develop into complete plants which later become independent when the internodes rot away. Such stems are termed creeping shoots. In some cases, strawberry, violet, pansy and cinqucfoil, these are modified into runners, of which the strawberry affords the best example. The parent plant has its leaves on a very short main stem. Towards the end of the flowering season, buds in the axils of these leaves grow outwards horizontally, usually with a compact terminal bud and a node bearing only scale leaves. The internodes grow to several inches in length. The terminal bud turns upwards, produces adventitious roots underneath and develops into a complete new plant. Quite often the runner continues as an axillary shoot of this new plant. The gardener either selects plants which normally produce runners after the fruit or cuts them away should they appear too early, for they take the materials which would otherwise go to form large strawberries.

The houseleek and London Pride produce runners with short internodes, so that the new plants form a clump around the original plant. These are termed offsets.

Suckers, which are underground shoots, may be produced from underground stems, or, more rarely, from roots. Mint forms a complex network of suckers just underneath the soil which, although white, are recognizable as stems by their possessing scale leaves ; the terminal buds turn upwards to grow into the aerial portions. Fruit trees such as damsons and apples have a tendency to produce suckers, especially after severe pruning. These, like those of the rose, make demands upon the plant which interfere with its production of fruits and flowers respectively. Bindweed, sheep ’s-sorrel, horse-radish, lilac and raspberry produce adventitious shoots from their roots which, in the case of the field plants, renders them obnoxious weeds, for they are difficult to eradicate, especially in the case of horse-radish. The new shoots of the raspberry produce flowers the next year, so the old canes are usually cut away when their leaves show signs of dying off.

The common bramble exhibits a curious mode of vegetative reproduction. Long woody shoots grow out and bend over until they touch the ground a few feet away. The terminal bud becomes swollen and whitish, produces adventitious roots and a mass of leafy shoots which grow into a new bramble bush. This mode of reproduction has proved so sucoessful that the spreading of the plant is becoming a serious menace to the farmers of New Zealand, where the plant has no natural enemies such as insects tending to restrict its growth, for it was introduced into the country from England.

Man has discovered various ways in which he can multiply plants conveniently to suit his purposes, thereby saving much time, trouble or expense. Cuttings when placed in warm moist soil, generally in autumn, produce adventitious roots at the lowest node, and are ready for planting out the following spring. Gooseberry, currant, pelargonium, fuchsia, antirrhinum, calceolarias, violas and pansies of various kinds, as well as root cuttings of peony, are commonly propagated in this way and ’require far less time than would be necessary to grow them from seed.

Begonias may be propagated by leaf cuttings. The principal veins on the under surface of thick leaves—preferably not too old—are cut and the cut surfaces—as with all cuttings—should be allowed to dry before further treatment. A waterproof corky layer or callus forms over the cuts, reducing water loss, and, what is more important, preventing infection by fungi. The leaves are then kept firmly pressed against warm and moist sand. Adventitious roots appear just above each callus and later tiny shoots. Once established, these may be separated from each other to pursue a separate existence.

Carnations and pinks are usually propagated by pegging down or ’layering ’a shoot and cutting along a node, where it touches the soil, causing it to produce adventitious roots there, ultimately to become a separate plant. More highly skilled methods are those of budding and grafting. In both cases the object is to cause a variety having some desirable quality to grow upon a well-established root system. Budding, as, for example, of roses, which do not readily grow from cuttings, is usually carried out in dull showery weather in June or July. When the scion has finished flowering a dormant bud is so cut from it that it has attached to it a flat, narrow, pointed, shield-shaped piece of the stem, the edges being smooth. The leaf blade can be cut off, and the woody part below the shield cut away with a sharp knife, leaving the cambium exposed. The bark of the stock is opened at the same time by a T-shaped cut, the edges of which are turned back with the knife to expose the cambium layer. The shield is slid down the exposed part of the stock, and its top cut so that it fits into the horizontal part of the T-shape. This is then firmly, but not tightly, bound into position by bast or soft wool, and may finally be made waterproof with a suitable covering. If the two cambium surfaces are in sufficiently intimate contact, the tissues will grow together and the bud will shoot the following spring.

Grafting consists of binding a scion tzvig to a stock. The way in which the cut surfaces of scion and stock are arranged varies. It may be a scion which is tongue-shaped to fit an exposed surface, or a wedge to lit a V-shaped cut, or—this commonly proves to be most sucoessful— an N-shape, with a stock cut to correspond. In each case sucoess depends upon the cambium surfaces being brought closely and permanently together, with complete protection and support of the two parts which are usually bound together and then rendered safe from infection, e.g. by fungi, and from drying up by grafting wax. The most suitable time for grafting is in the spring when the plant is normally growing very rapidly.

Generally scion and stock are different varieties of the same species, but they need not be ; apple twigs may be grafted on to pear trees, and peach on to plum. This illustrates two points : that the protoplasm of the scion is different from that of the stock, retains its individuality and does not become like the stock, and that the protoplasm of the scion is capable of building up material peculiar to itself with the food supplied by the slock.

It is interesting to note that many of the plants which exhibit vegetative reproduction or which can be artificially propagated by man are either monocotyledons or belong to the natural order Rosacea; of the dicotyledons.

The Advantages of Natural Vegetative Reproduction.

The new plants exactly resemble the original plant from which they grew, so that the advantageous variations possessed by it are exhibited unchanged.

The extent to which it occurs is dependent upon good conditions, and is to some extent independent of the life cycle, I.e. it is generally caused by good conditions and not by a certain stage in the plant ’s life, so that since both food and buds are present development can be very rapid.

It is independent of such processes as pollination and dispersal, which are not bound to occur since they commonly require some external agency such as wind or insects to bring them about; it depends upon the plant ’s own activity.

In certain cases, such as those of runners, large areas can be covered in a relatively short space of time.

The new plants can obtain nourishment from the parent until they are established, and so are able to survive temporarily unsuitable conditions ; unlike germinating seeds with their limited amount of food.

The Advantages of Artificial Propagation to the Horticulturist.

He can perpetuate and multiply varieties having some desired peculiarity, e.g. flavour of apple, colour of rose, since the scion retains its peculiarities.

He can modify the period of maturity, e.g. early maturing apples are grafted on ’paradise ’stock and later maturing apples on ’crab ’stock.

He can obtain flowers or fruit one, or at most two years, after budding, etc., instead of waiting several years for the plant to develop to the fruiting stage, which is the case when grown from seed.

In general, such plants once established require less care and attention than young plants grown from seed.

Some plants—banana, nectarines, ’seedless ’ oranges and certain roses—do not produce seeds, so that artificial propagation is the only means of perpetuating and multiplying them.

The Disadvantages of Natural Vegetative Reproduction.

Overcrowding can soon occur, so that

There is a competition for light and for raw materials from the soil; consequently the plants tend to be choked and small.

Since there is a lack of variation, the plants are not so adaptable to changing conditions as those which are produced from seed.

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