Methods Of Vegetative Reproduction And Propagation
( Originally Published 1915 )
Tubers. Bulbs and corms. Bulbils. Runners. Layerings and cuttings. Budding and grafting.
As explained in my first lecture vegetative reproduction is the multiplication of the plant by means of structures, which partake of the nature of vegetative organs and are not the result of the fertilisation of flowers.
This means therefore that the offspring, however numerous they become in the course of years, are not so much descendants as actual portions of the original individual just as the most recently formed buds of a tree are really part of the same tree, which ten or a hundred or even a thousand years before bore similar buds. The only difference is that the buds which the tree produces year after year all remain attached to the same parental stem, while in the case of tubers or bulbs the parent plant dies down each year, so that these structures become so many separate individuals. The very large number of distinct plants which thus arise will perhaps best come home to us if we consider that in the case of a potato plant producing yearly only six new tubers, we should have at the end of ten seasons obtained over ten millions of tubers.
If an average of ten tubers were produced by each plant, the number of tubers at the end of ten generations would be 10,000,000,000 potatoes. As the millions of new plants which are thus developed are, properly speaking, all parts of the same plant they should, apart from slight differences. of nutrition, not only be all similar but actually of the same constitution. It is true that bud variation does occasionally arise in plants, but it is a comparatively rare phenomenon, and consequently we find that as a rule plants, raised by vegetative methods, maintain the character of the parents and do not show the sporting with which one is familiar when raising plants from seed. The purity of the strain is thus easily maintained by vegetative propagation.
Let us now examine some of nature's methods of vegetative reproduction. In the first place, we have such well-known examples as tubers which form the chief method of propagation of the potato and of the Jerusalem artichoke. A tuber is the swollen-up end of an under-ground branch. This can be clearly seen by digging up a young potato plant and following these subterranean shoots from the parental stem to their tuberous tip. Both the branch and also the tuber show the rudiments of leaves, reduced to small scales and spirally arranged round the potato. On an old potato the sickle-shaped mark below each eye or lateral bud represents the insertion of the leaf or leaf scar, and at one end of the potato where these become more crowded together, we have the terminal bud of the tuber. This is called the "rose" end. At the opposite end we can generally find the scar or heel where the branch of which the tuber is the dilated tip., was joined to it. In all cases tubers are filled with stored food material, mainly starch in the case of the potato. When potatoes are set in the spring, the terminal bud grows up to form the main stem of the new plant, while the other eyes remain generally dormant. The new tubers are formed from specialised underground branches produced from the lower region .of the main stem. The latter is therefore often earthed up so as to promote the development of these side shoots. It is a very common practice before setting potatoes, to place them side by side with the "rose" end uppermost and allow the terminal shoot to commence its development in daylight. By this means the shoot does not elongate so rapidly as when grown underground, that is, in the dark, and thus the lateral buds, which will develop into tuber producing shoots, are more closely crowded together and more numerous. This method therefore results in a greater yield of new tubers.
As there is generally a superabundance of food material, it is possible to divide a tuber into several pieces, and provided each piece has enough food material and a sound " eye," it will produce a new plant. It has been found by practice that when cut for " sets " the pieces near the " rose " end give the best results. When a potato is divided up in this way it is necessary to leave the separate pieces spread out to dry before they are planted, so that they may form a protective layer before coming in contact with the soil, the bacteria of which might cause them to decay.
Recently a method of raising new potatoes in the dark has received prominence in the press, and in many quarters erroneous views have been formed concerning this mode of cultivation. It has been thought by some that potato plants could grow like mushrooms and were not dependent upon light for their full development. This is very far from being the truth. Of course it is well known that potato tubers will sprout in the dark, as they do indeed in nature underground, but they are only able to grow without light while there is still a supply of food material in the old tuber upon which they can draw for their development. When that is exhausted the plants must inevitably die, as without light they are Iike all green plants unable to manufacture new organic material. It has however been found by experiment that if tubers are then half buried in fine dry soil spread out on a table in the cellar, they will soon be surrounded by a crop of small new potatoes close up to the old tubers. Large tubers should he used for this method of cultivation, and they should be placed three or four inches apart. It must of course be remembered that these small new potatoes are produced entirely at the expense of the food material stored in the old potato. We are therefore by this method of cultivation not increasing the food supply of the country but merely replacing old potatoes by a crop of more palatable new ones.
Bulbs and corms, which are the vegetative methods of reproduction of many members .of the Lily and Iris Families, must be looked upon as specialised underground buds, which by virtue of their store of food material are able to lead an existence independent of the plants on which they have been produced. Like the ordinary winter buds of a tree, such as the horse-chestnut, they are protected on the outside by a few dry scaly leaves, while at their centre will be found the foliage leaves and flowers of the next season. But between these we find another set of leaves which do not occur in the buds of a tree, namely, thick storage leaves of a fleshy nature. It is the possession of this internal supply of food material which enables these specialised bulbs to produce their leaves and flowers in the next season, though separated from the parent plant which formed these bulbs, while in the case of the tree the winter buds expand in spring by making use of the food material stored in the branch to which they belong.
What is the nature of the parent plant on which the bulbs are borne as lateral buds? Take up a tulip plant after it has flowered and you will find at the base of the upright stein bearing the I eaves the storage scales which have made the growth of the stem and leaves possible, and at the base of one or more of these scales will be seen small buds, which are beginning to swell owing to the organic material manufactured by the leaves, passing down to them. This observation will teach us that it is important, if we wish to save our own bulbs for planting, to leave the old bulbs in the ground for some time after flowering and not to cut off the foliage leaves but to pre-serve them as long as they are fresh and green and capable of manufacturing food material. This is the time, too, to give the plant further food in the form of top dressing or occasional supplies of manure water. It is also important to cut away the dead flowers, so that the food material is not expended in the ripening of the seed vessel.
Another point of importance in bulb growing is to secure the proper ripening of the bulb. In nature bulbs after Iosing their leaves pass through a resting stage, during which they are kept dry by the vegetation covering the soil in which they grow, whether they occur in woodlands or meadows. In our gardens where the ground above them is usually uncovered, water percolates down to them, and if they are not very deeply buried slugs, too may attack them. It is therefore often advisable in. the ease of damp soils, to take up the bulbs when the leaves are dying away, and to allow the bulbs to dry in the sun, storing them afterwards in a dry place until the time for planting arrives.
Corms are very like bulbs and may often be mistaken followed by fleshy storage scales all the food material being stored in the thicks bulk of the corm. In their however, they follow very closey dry scales which surround them on the outside are for the latter, but on cutting across we find that bulb. Crocuses and Gladioli possess corms, while Lilies, Tulips, Hyacinths and Daffodils have true bulbs. The bud-like nature of bulbs can be clearly recognised by examining the small swollen buds or bulbils which arise on the aerial stems of certain species of Lily, and which, though unable to give rise immediately to a new flowering shoot, can gradually be grown on to produce in time a mature bulb.
Runners, such as are produced in the Strawberry and Violet, are delicate lateral shoots creeping over the surface of the soil and becoming readily- rooted at their nodes when in contact with the moist soil. In nature they cause the very rapid spreading of these plants, and by the decay of the portion of the stem which joins them to the parent plant they may cause the increase of individuals. They are conveniently used for propagating purposes and should always be removed from the parental stock as they draw nutriment from it and therefore impoverish the latter, with the result that they reduce the number of flowers.
Many plants which do not possess natural means of vegetative reproduction can be caused to give rise to new individuals by separating certain portions, generally lateral shoots, and inducing the same to develop new roots. In some instances the formation of these roots is promoted before the branch is separated from the plant; this process is known as layering. The lower branches of such shrubs as Gooseberries and Red Currants may be bent down and partially embedded in a shallow trench dug round the bush, and filled with light and porous soil. When stimulated by moisture the buried portions of these branches, the tips of which must be allowed to project beyond the trench, will produce what are termed adventitious roots, and when these are sufficiently well-established, the branch may be severed from the parent plant, and the new individual will lead an independent existence. It is generally found advisable to cut back the projecting portion of the branch to two or three buds. This method can also be employed for Rhododendrons and for more delicate shrubs, as well as for climbers like Clematis. A similar process has been found advantageous for many herbaceous plants such as Violas, Carnation and Pinks. In these cases, however, it is usual to make a slight incision in the buried portion of the shoot, which should always extend to one .of the knots or joints. Such an incision stimulates the production of roots, particularly at the end furthest removed from the parent plant. This is no doubt due to the accumulation at this point of food material which has been produced by the leaves at the tip of the layered shoot, the food material being unable to be carried across the incision. Even in dealing with shrubs it is sometimes found advisable to make an. incision partially across the branch which is to be layered, and to peg it down so as to keep it in position until the new roots are formed. It is of great importance that the soil in which the new roots are to form should be well aerated, more particularly in the case of herbaceous plants which are more liable to injury by rotting. It is advisable, therefore, to add a considerable amount of sand to soil in which layerings are to be embedded.
In the case of upright stems or branches, which are far above the soil, the production of adventitious roots can be promoted by removing a ring of outer tissues to the depth of the wood, and then tying a handful of wet moss round the shoot. Such ringing causes the food material formed in the upper part of the plant to accumulate above the wound, and this promotes the rapid development of roots. When these have become sufficiently established the rooted portion of the stem or branch may be severed from the parent.
In many plants the formation of adventitious roots is so rapid that shoots completely detached from a plant can establish themselves as cuttings, producing their own roots when placed in suitable conditions. In all cases in which cuttings are made the first need is to cause the covering in, that is, the healing of the cut end of the shoot. This is done by the development from the actively growing cells of a peculiar wound tissue, called callus, which by growing over the wound and developing a layer of cork, prevents the destruction of the exposed cells and the entrance of harmful bacteria. The growth of callus is promoted by the aeration of the tissues, and it is there-fore important that the soil in which cuttings are placed should be even better drained than that supplied to normal plants. A soil made porous by the plentiful admixture of sand should always be selected, and in the case of cuttings which arc being struck in pots, it is a common practice to insert them close against the inside of the porous pot, through which the air has a free access to the cuttings. The danger of over watering cuttings is even greater than in the case of well-established plants. Cuttings taken from woody plants produce their wound tissues as well as the adventitious roots, which are formed later, at the expense of the food material which is stored in the shoot. They are not therefore so dependent on warmth and light in the early stages as arc herbaceous cuttings. They can indeed be struck in the autumn or winter after the leaves have fallen, or in the early spring before the foliage has been developed. If taken in the autumn they often do not produce their roots until the following spring, and they are always later in the development of their leaves than are well-established plants. In the case of herbaceous cuttings which have no, store of food material, it is necessary that they should be able to continue to form new organic food material in their leaves so as to promote the growth of callus and the development of roots. It is esential therefore that they should have plenty of light; but in the first few days before they have adapted themselves to their new conditions, they arc liable to lose too much water by evaporation, and it is important during this period to keep them slightly shaded, or to grow them in a moisture-laden atmosphere in a closed frame or greenhouse. It is better to prevent: the loss of too much water by protecting the leaves in this way than by excessive supply of water to the soil, as herbaceous plants are very liable to decay by the action of bacteria on the cut end of the shoot. As herbaceous cuttings have to continue to manufacture food material, they also require a greater amount of heat than do woody cuttings, the process of leaf nutrition being stimulated by an increase of temperature. Herbaceous cuttings must, therefore, not be taken too late in the autumn unless they are to be grown in artificial heat.
A few plants can be raised from root cuttings. This is possible where plants are endowed by nature with the power of forming adventitious buds on their roots. Raspberries, Pears and Apples are all examples of plants which often produce suckers from their roots in the neighbourhood of the parent plant. Such growths can be separated and developed into new individuals. In the case of Raspberries indeed, this is a common method of propagating the canes.
Lastly, it has been found possible in the case of some plants with somewhat fleshy leaves to cause these, or even portions of a leaf, to produce adventitious buds. This is the case with many Begonias, particularly those belonging to large-leaved varieties. If the leaf is placed on damp soil, the midrib having been cut in several places, new plants may arise from each portion as with the stimulus of warm temperature and moisture the leaf produces a considerable growth of callus, from which adventitious buds soon arise. The fleshy scales forming the bulbs of most lilies are capable when separated from the parent bulb of producing small adventitious buds from which new plants can be grown, and this is a common method of propagating the plants.
Space prevents a detailed discussion of the processes of budding and grafting, but from the botanical point of view the processes may be regarded as a special case of making cuttings in. which the latter, instead of being planted in soil, are inserted in the tissues of a nearly related plant with which they become united by the development of wound tissue or callus. No adventitious roots are formed by the graft as the scion relies for its supply of water entirely upon the roots of the stock.
There has been much discussion as to whether, as a result of grafting, there is any influence of the stock upon the scion, or vice versa. A considerable amount of information on this question has accumulated, but it is largely of a negative character, and what positive evidence exists is of a doubtful nature.