Grafting - Comparison with cuttings - Effects of environment - Relations between scion and stock - Variation in grafts - Grafting and parasitism - Infection - Pollination - Grafts - hybrids - Predisposition of Natural grafts - Root-fusions.
Grafting is a process which consists in bringing the cambium of a shoot of one plant into direct union with that of another, and is practised in various ways, the commonest of which is as follows: One plant - the stock-rooted in the ground, is cut off a short distance above the surface of the soil, and a shoot from the second plant - the scion-cut off obliquely with a sharp knife, is inserted into a cleft in the stock, so that the two cambiums (and sometimes the cortex and pith of each as well) are in close contact: the scion is then tied in position, the wounds covered with grafting wax, and the whole left until union of the tissues is completed. This union depends on the for mation of callus at the cut surfaces, and the intimate union of the ingrowing cells from each callus.
The development of the callus follows the course described for wounds, cuttings, etc., and the union is exactly comparable to the union of the two lips of a healing callus over a wound (see p. 197).
Grafting was known and practised far back in the ages. Virgil was well acquainted with the process, and Theophrastus compared it with propagation by cuttings.
The scion differs from a cutting, however, in having no roots of its own: it is parasitic upon, or rather is in symbiosis with the stock, the root and tissues of which intervene between it and the soil. Consequently the selective absorption, size and number of vessels, and innumerable other physiological and anatomical peculiarities of the stock determine what and how much shall go up into the scion, while the latter supplies the former with organic materials and rules what and how much food, enzymes, and other secretions, etc., it shall receive to build up its substance. Surely, then, if such factors as the nature of the soil, the water and mineral supplies, the illumination, and the various climatic factors of altitude can cause variations on a plant direct, these and other factors are still more likely to be effective on stock and scion, and each must affect the other.
Nevertheless opinions have differed much as to whether any important effect is to be seen, and on no point more than on whether the scion can affect the stock, in spite of such examples as Cytisus Adami, Garreya on Aucuba, Sunflower on Jerusalem Artichoke, etc. Recent results, especially of experiments with herbaceous plants, show that not only can the stock affect the scion (and vice versa) directly, but the effect of the changes may be invisible on the grafted plant and only show itself in the progeny raised from the seed of the grafted plant. In other words, variation occurs in grafts either directly, as the results of the effects of the environment on the graft, or owing to the interaction of scion and stock, showing as changes in general nutrition in the tissues concerned, etc., owing to special reactions of the protoplasm of the uniting cells one on the other, and of the results of the further protoplasmic secretions, sortings, and so forth, on the cells developed as descendants of these in the further growth of the graft; or indirectly, in that some of these changes so alter the nature of the special protoplasm put aside for reproductive purposes, that the resulting embryo in the seed transmits the effects, and they show as variations in the seedling. If these results are confirmed they should meet all objections that have been urged against the transmission of acquired characters.
In fact there are analogies between grafting and parasitism which cannot be overlooked, and should not be underestimated, their commonest expression appearing in the alterations in stature, habit, period of ripening, and so forth. These analogies are easily apprehended when we compare parasites like the Mistletoe, Loranthus, or even such root-parasites as the Broom-rapes and the Rhinanthoideae with grafts; but they also exist in the case of many fungus-parasites, and we might almost as accurately speak of grafting some fungi on their hosts as of infecting the latter with them, especially when it is borne in mind that the effect of the scion on the stock is by no means always to the benefit of the latter, and that there are reasons for regarding the action of some such unions as that of a sort of slow poisoning of the stock by the scion. Why do we not here say that the stock has been infected by the scion?
The resemblances between pollination and the infection by fungus hyphae may also be insisted upon. If we take into account Darwin's remarkable experiments showing that in "illegitimate unions" the pollen exerts a sort of poisonous action on the stigmas or ovules, it is possible to arrange a series of cases starting with perfectly legitimate pollinations where the pollen tube feeds as it descends the style on materials provided by the cells, and proceeding to cases where the pollen is more and more merely just able to penetrate the ovary and reach the ovules, to the extreme cases where no union at all is possible.
Side by side with such series could be arranged analogous cases where fungus spores can enter and infect the cells of the host, and live symbiotically with or even in them, or can penetrate only with difficulty, or with poisonous effects, and finally cannot infect the plant at all.
Less obviously, but nevertheless existing, are gradations in grafting to be observed, where one and the same stock may be successfully combined with a scion which improves it - or which is improved by it - or the scion may unite but acts injuriously on it, or, finally, cannot be induced to unite.
But we may go further than this in these comparisons. Just as the results of pollination frequently induce far-reaching effects on distant tissues - e.g. the swelling of Orchid ovaries, and rapid fading of the floral organs - so also the effects of hyphae in the tissues may induce hypertrophies, deflection of nutrient materials, and the atrophy of distant parts - eg. the curious phenomena observed in Euphorbia attacked by Uromyces - and some of the distant actions in grafts may be compared similarly.