Part 19 (1/2)

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 alt.i.tude 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 a.n.a.logies 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 a.n.a.logies 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 a.n.a.logous 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--_e.g._ the curious phenomena observed in _Euphorbia_ attacked by _Uromyces_--and some of the distant actions in grafts may be compared similarly.

Going still further, we may compare the effects of cross-breeding or of hybridisation, where the _progeny_ show that changes have resulted from the mutual interactions and reactions of the commingled protoplasm, with Daniel's results, in which he obtains proof of such interactions of the commingled protoplasmic cell-contents of grafts in the seedling progeny; although there is no probability--we may even say possibility--in this latter case that the effects are due to nuclear fusions, but only that the germ-plasm of the seed-bearing plant has been affected by the changes in the cell-protoplasm which nourishes it when the reproductive cells are forming.

In the case of graft-hybrids the matter appears to be somewhat different, and we may well suppose, with Strasburger, that the commingling of characters observed in flowers, fruits, foliage, etc., on shoots borne after grafting are due to the occurrence of nuclear fusions during the union of the grafted tissues; though it is by no means impossible that what has really happened is profound alterations in the nuclear substance (germ-plasm) owing to its being nourished by cell-protoplasm (somato-plasm) which has been itself affected by the interchanges of substance between scion and stock, and therefore itself furnishes a different nutrient medium from the unaltered cytoplasm of either.

But even here we can find parallels among the ordinary phenomena of plant reproduction. Maize plants with white endosperm containing starch, if crossed by pollen from other plants with purple endosperm containing sugar, bear seeds with purple endosperm containing sugar, and such _Xenia_ may be compared to graft-hybrids in many respects.

I know of no case among fungus infections which could be compared directly with these examples, and it is not at all likely that we shall meet with any instance of a fungus-hypha handing over nuclear substance to an egg-cell, and so affecting the latter that an embryo results. But the case is not hypothetically impossible, although the distant relations.h.i.+ps of the two groups of organisms render it extremely improbable among the higher plants. It is by no means so improbable, however, that further research may show cases where the egg-cell of a lower cryptogam--_e.g._ another fungus--may be affected either directly, or indirectly, by the protoplasm of a parasitic or symbiotic hypha, as suggested by the extraordinary phenomena of symbiosis.

Some of the variations in grafted plants are found to predispose the plant to disease, or the reverse, and cases may be cited where the resulting shoots, foliage, or fruits, or seedlings more readily fall a prey to, or resist, parasitic fungi and insects than the ungrafted plants. Daniel gives instances of such--_e.g._ among other examples, Peas grafted on Beans yield seeds which suffer more from Erysipheae than the normal seedlings. But the best known cases are those of Vines in their relations to _Phylloxera_, already referred to (p. 155).

Several instances are also known where grafted plants show more or less resistance to such factors of the environment as low temperatures; grafted or budded Roses often suffer much from Erysipheae, and so forth.

Much research is still needed to determine how far these matters depend on real alterations in the nature of the graft, or _are only true for the localities in which the experiments have been made_, a point which has, I think, been overlooked by all observers.

Grafted plants are apparently very much exposed to injury by slugs, insects, and the invasions of parasites during the healing of the callus and the fusion process. Here again it must not be overlooked that the callus is, so to speak, a t.i.t-bit of luscious, thin-walled, succulent tissue; and, like all wounds, the graft affords entrance to parasites such as _Nectria_ and Ascomycetes of various kinds, under circ.u.mstances very favourable to their invasion.

_Natural Grafts._--It is by no means an uncommon event to find the branches of Beeches, Limes, and other trees which have been accidentally brought into contact during growth, joined where they cross. As they press one against the other, they become naturally grafted, by that form of the process known as _inarching_: except that in artificial inarching the operator cuts off the cortical tissues of the two branches and brings their cambial surfaces together, whereas in nature the cambiums only come into contact after the destruction by pressure, or slight abrasion, of the entrapped intervening tissues. The fusion occurs, in fact, exactly as in the burying-in of a nail or wire, referred to on p.

211.

Natural grafts are very common among the roots of trees, and possibly explain some queer cases of the apparent revivification of stumps of trees not usually given to forming abundant stool shoots. It is regarded as probable in some old forests that the majority of the roots of trees of the same species are linked up together by such natural grafts, a probability not diminished by the fact that such roots cross at many points, and are easily grafted.

NOTES TO CHAPTER XXIX.

The student should read Bailey, _The Nursery Book_, 1896, for details regarding the practice of grafting, and facts in abundance can be obtained from the pages of the _Gardeners'

Chronicle_.

Concerning graft-hybrids and the variations of grafted plants see Jouin, _Can Hybrids be obtained by Grafting?_ and especially Daniel, ”La Variation dans la Greffe,” in _Ann. des Sc. Naturelles_, S. VIII., Vol. 8, 1898, p. 1, and the literature there collected. The whole subject is largely controversial, and much work remains to be done.

CHAPTER x.x.x.

LIFE AND DEATH.

_Protoplasm--Hypothesis as to its structure and behaviour-- a.s.similation--Growth--Respiration--Metabolism--Action of the environment--Nuclear protoplasm--Pollination--Grafting-- Parasitism--Graft-hybrids--Life--Death--Variation--Disease._