Before adverting to the artificial process as practised by the gardeners, it may be well to allude to what Nature herself does in this way without assistance from man. The union of branch to branch of the same tree is so common a phenomenon that we need not dwell upon it further than to note it as the simplest and commonest case of grafting, at least so far as flowering plants are concerned. Among the fungi, indeed, or even in the early stages of growth of the mosses, the young plants become so inextricably intergrafted that the so-called individual is really a republic one and undivided. In the higher plants the grafting process is exceptional, and is the result of some abrasion which removes the outer rind, and thus allows the growing tissues of the two abraded surfaces to come into contact, and under favourable circumstances to adhere to each other. Union of the contiguous branches of two trees of the same species is of equally common occurrence with that just mentioned, and to this occurrence the great size of some trees is attributable. We mention these more familiar illustrations with nothing more than passing comment. They illustrate the power that growing vegetable tissues have of uniting, and that is all we want with their testimony in this place. More important for our purpose is the evidence that plants of different species will unite together. This has been denied, but there are plenty of cases on record, and one facetious observer (Charles Waterton) compared the union of a spruce-fir with an elm, and the consequent stunting of both, to the incongruous union of Church and State! Such cases are certainly abnormal and exceptional, but they exist nevertheless, as a visit to Richmond Park will attest. There may be seen, or might have been a year or two since, a thorn (Crataegus) adherent to a horn-beam (Carpinus). There are cases where the contact of the two trees has been so firm and so persistent that at length the two have become actually inseparable unless great force were used. It must, however, be remembered that we cite these cases simply as instances of the union of two distinct species, not of grafting properly so called. The difference is this a graft derives its nourishment through the stock on which it is placed, while in the cases just alluded to each plant, though firmly joined to its neighbour, is perfectly independent of it in the matter of food. The same statement, however, cannot be made with reference to the mistletoe or the Loranthus. These are different enough from the trees on which they grow; they adhere to their fosterparents with a tenacity greater than that of any graft, and they suck the very life-blood out of them, ensuring their own destruction by causing the death of the trees on which they grow. It is worth while noting this fact in connection with the well-known tendency that grafting, as artificially practised, has of shortening the term of life of the plant. Other cases of natural union are worthy of remark, especially the union that sometimes takes place in roots. For many years it has been known that the stumps of silver-firs increased in diameter after the trunks had been felled. Here was a pretty case for those who held the presence of leaves as an essential to the due formation of wood. How would they get over this difficultythat wood there was, and yearly increasing, and yet no leaves? Even quite recently one of our agricultural societies has awarded its prize to an essay in which the phenomenon in question is in some way or another explained by the antiseptic action of peat! What a delightful discovery! Would that the salt-beef in the brine-tub would increase in like manner! Jesting apart, the cause of the annual growth of the stumps of the silver-fir was satisfactorily shown some twenty years ago by the German botanist Goeppert. He was enabled to prove that the roots of the felled tree inoculated with those of adjacent trees, and that a communication of the nutrient fluids from the sound tree served to keep life in the maimed one. Doubtless a similar root-union exists in other cases, and affords the explanation of the formation of those seemingly detached knobs of oak that one occasionally meets with. * Another instance of root-union is worth mention, not only for its inherent singularity, but because it will yield us important evidence by and by. We allude to the case of the red and white carrot recorded by Lindley. The two roots by some means became twisted one around the other and firmly united together. But this was not all. While the tops or crowns of the two carrots preserved their natural appearance above the point of union, it was very different below. In fact the characteristics of the roots below the union were exactly transposed. What should have been a red root became white, while the white root blushed with a redness not its own. We may illustrate what happened in the case of these carrots by the letter X, consisting as it does of two lines, one thick the other thin, crossing in the centre. Now, suppose the thick line to become thin below the junction, and the thin line to become thick, and we shall have a change analogous to that which took place in the carrots aforesaid. Another curious phenomenon occasionally met with is the union of embryo to embryo, either within the seed or immediately after germination. In most cases a seed contains but one embryo plant, but there is always a provision made for more than one, and in fact sometimes two or more are produced, as in the orange (Citrus). The mistletoe is one of these "Ann. Sc. Naturelles," xix. 1843, p. 181, t. iv. plants, apt to produce twin embryos, and, what is more to our point, the twain are not unfrequently adherent like their famous Siamese counterparts. We have before us as we write, thanks to the courtesy of an American correspondent, a case wherein two seedling plants of the Osage orange (Maclura) are thus united together. In this plant the seedling consists of a root or radicle, surmounted by a "caulicle" which bears the two seed leaves above which the stem proper begins. Now, in our specimen, the roots are free and the stems are free, but the two caulicles are intimately united throughout their entire length. In America, where the Osage orange is largely grown as a hedgeplant, such unions are said to be not infrequent. Mr. Thwaites, the eminent director of the Botanic Gardens, Ceylon, records a yet more curious instance, wherein two embryos were contained in one seed of a fuchsia, the two embryos possessing, moreover, different characteristics-a circumstance probably due to their hybrid origin, the seed in question having been the result of the fertilisation of one variety of fuchsia by the pollen of another. * It would be easy to multiply instances, but we have said enough to show that union may, and does occasionally, take place between different parts of the same individual plants, or between different plants of the same species, and even between plants of different specific nature. Gardeners have not been slow to avail themselves of this hint. At this season of the year, in our large nurseries, a small army of expert workmen may be seen preparing the stocks for the reception of the "graft," adjusting the latter in its place, and with an amount of precision, dexterity, and rapidity truly marvellous, the more so as a glance at the horny hands of the operators would not lead one to credit their owners with the possession of the requisite surgical nicety of manipulation. One main object of this grafting process is the multiplication of desirable varieties of fruit or other trees, which could not be reproduced by other means with sufficient certainty and rapidity, and in some cases not at all. Other reasons why grafting is done will become apparent as we proceed. In the meantime, we may briefly allude to some of the conditions for successful grafting, so far, at least, as they are yet known to us. first is that the plants furnishing the stock and the scion respectively should be nearly related one to the other. We may set aside as fables the stories previously alluded to, or at any rate we may explain them by the operation of causes other than those of grafting properly so called. But there is something more than mere botanical kinship necessary, and what "Ann. Mag. Nat. Hist.," March 1848. The that is is at present in great degree a mystery. It is readily intelligible that there must be a certain conformity of habit between stock and scion, that the two must be well matched as regards vigour, health, time of starting into growth, and the like, that the tissues of the plant must be sufficiently alike to permit of due contact and union, and so on. But these facts will not suffice to explain the sympathies and antipathies which plants manifest. A pear (Pyrus) will graft on another pear, on a quince (Cydonia), or on a hawthorn (Crataegus); but there is difficulty in getting it to grow on an apple, and a like difficulty in inducing an apple to grow on a pear, closely as the two are related. Cultivators are often sadly puzzled to find a suitable stock on which to "work," as they phrase it, some desirable variety, and it is only by repeated trials with various plants that they succeed. In such cases they have nothing to guide them but the general principle that there must be some near botanical affinity, and, as we have just seen, even that fails them occasionally. For years it was a hard matter to find a stock on which Viburnum macrocephalum could be grafted, in spite of there being plenty of near relations at hand. On the other hand, the Loquat (Eriobotrya) will graft on the pear, the Eriostemon on the Correa, genera which, under the circumstances, we should not call very closely allied, while, in numerous instances, evergreen plants will graft on stocks of deciduous plants. A perennial species of convolvulus grafted on an annual species has caused the latter to assume the perennial habit of the scionnay, some French nurserymen have even succeeded in engrafting a bud on a leaf. Not only did union take place, but the leaf thus made to serve as a stock instead of speedily perishing, as it would have done under ordinary circumstances, acquired a greater degree of permanence-assumed, in fact, the characters of a stem.* It is evident, then, that much yet remains to be learnt as to the why and wherefore of these sympathies and antipathies. In addition to a certain not remote botanical affinity, and to conformity of physiological conditions, it is obvious that nice adjustment and accurate contact of the growing tissues must be secured and maintained if the graft is to be satisfactory. "On each lopp'd shoot a foster scion bind: "Gardeners' Chronicle," 1866, p. 386. A close paraphrase, on the part of Erasmus Darwin, so far as the last lines are concerned, of those of Virgil, already cited. Turning now to the effects produced by grafting on the scion and on the stock respectively, we open up a very interesting subject for enquiry, and we make apparent the objects for which grafting is employed. Gardeners, as a rule, hold that in the great majority of instances no effect beyond adhesion is produced. There are some plausible reasons for this opinion, it must be admitted, inasmuch as the change is very often not obvious on the surface. One experimenter tells us for instance that he grafted at various times on the same jargonelle pear no less than eighteen different grafts. Of these eighteen, ten were apples of various kinds, while the remainder were made up of pears, hawthorns, medlar, and quince. All these grafts, we are told, succeeded-at least for a time; fruit was produced from the scions in nearly all cases; but there is no evidence to show that this extraordinarily composite tree ever produced any fruit differing from the usual character of that naturally yielded by itself, or by its numerous parasitically attached grafts. More extraordinary still is a case wherein a French experimenter, M. Carillet, of Vincennes, first of all took two peartrees, both of which were grafted on the quince stocks. These we will call A and B. A was planted in the usual way, then B was grafted on it, but in an inverted position, head downwards, roots uppermost. When the operation was completed, there were thus two pear-trees united by their leading shoots, but the upper one, B, was reversed in position, with its roots completely exposed. To add to the strangeness of the experiment, M. Carillet next grafted on the ends of four of the principal roots of B-quince-roots of course-four different varieties of pears, two of which succeeded; so that the entire plant consisted, first of a quince stock rooted in the soil and bearing a grafted pear; on this latter, but in an inverted position, was another pear, also grafted on the quince and with its roots uppermost; on these again were grafted two more pears. This illustration shows that the current of the sap is quite independent of the direction of the tissues. It must have passed as readily through the inverted as through the erect stem; but what is more to the point, so far as we are concerned at present, is that though the sap passed through no less than six different organisms, adherent one to the other, yet each portion of the composite structure retained its own individuality.* There are, indeed, many cases in which no apparent change takes place, as a result of grafting. Archbishop Whateley, with a view of ascertaining whether any change "Gardeners' Chronicle," 1867, p. 947, ex 66 Revue Horticole." |