Gardeners’ Chronicle, 1900 (Jan. 6, 12-13; Jan. 20, 35-36; Feb. 26, 85-86; Feb. 24, 116)

VARIATIONS PRODUCED BY GRAFTING, AND THEIR INHERITANCE

*Annales des Sciences Naturelles Botany. Series 8, vol. viii. (1898), pp. 1-226, pls. i.-x.

ALLUSION has already been made in these columns to Monsieur Lucien Daniel's contributions upon the subject of grafting.* The paper embodying the whole of his researches is, however, worthy of the more detailed consideration which we may now give to it.

Grafting, says Monsieur Daniel, has long been compared with the making of a cutting, and in so much as in both processes a shoot is caused to grow independently of the plant which produced it, the two are alike, but here the similarity ends. There can he no tendencies towards variation capable of manifesting themselves in the cutting which did not previously exist in the individual from which it was taken. On the contrary, in grafting, the author points out, there are many incitements to variation acting and re-acting upon scion and stock, in the shape of the altered circumstances in which both find themselves. Among these are changes in the amount and nature of the crude sap supplied to the scion by the stock, and of the elaborated food given in return, as well as the fact that either component of the new plant may begin or relinquish active growth independently of the other.

Furthermore it is argued, under such conditions of growth, the actual living matter of the plants is likely to be modified, and should this change be admitted, there arises a number of questions of no less importance to the horticulturist and agriculturist than to the scientific worker.

The problems to be solved deal with the extent of such variation as is due to grafting. Does this, it must be asked, show itself on the surface Are both, or is only one of the plants, affected? and are the modifications only such as would occur if normal plants were subjected to changes of nutrition in the ordinary course of events?

Again, are varietal or specific characters altered? If so, does the modification touch them permanently or for a time only, individually or collectively? Lastly, are only the bodies of the plants concerned, or are the pollen-grains and egg-cells acted upon, and will the new characters in the latter case be hereditary? The answering of these questions would appear to be simplicity itself, but unfortunately, it involves the exceedingly vexed question as to whether features acquired (luring the lifetime of an individual can be transmitted to its offspring, and we are brought face to face with a great diversity of opinion among practical men, botanists, and particularly philosophic naturalists.

For instance, some practical workers still regard grafting as belonging to the realms of the marvellous, believing, with the ancient classical writers, that all species can be radically changed at will. In this case some special influence must constantly be at work, and must make itself felt in a very important degree. Baltet says that grafting is federation in which both parties retain their own government.

E. A. Carrière and André consider graft hybrids to be an impossibility; but leaving practical men, Van Tieghem, summing up current opinion, says that grafting is a valuable method of fixing and preserving congenital variations, seeing that by this means further variation is prevented. Dr. Vöchting goes further, and relegates all the facts hitherto recorded on the influence of stock on scion, or scion on stock, to the category of old wives' tales, saying that no such special effect has been demonstrated. M. Daniel concludes this series of opinions by adding that Weismann holds all transmissible variations to have a sexual origin, a belief which Bailey does not agree with, basing his contention upon bud variations, while others from the animal side have come to similar conclusions.

It is allowed in the paper under consideration that those who join the swelling ranks of Weismann's followers have just cause in refusing to acknowledge the experiments advanced against their theory, for these are not by any means precise. Any further evidence, M. Daniel says rightly, must not be open to similar criticism, and the experiments upon the subject which he has been conducting since 1890 have been strictly comparative. He has always planted side by side with his grafts, in like conditions as to soil, climate, and so on, other "control" plants of the same varieties as the stock and scion. In this way variations due to external environment should be easily determined, and consequently those directly due to grafting.

After the point, that one positive fact may at once get rid of a mass of negative evidence has been emphasised, M. Daniel proceeds to discuss his results under a number of headings and detailed sub-headings.

Variations in the plants themselves maybe due either to altered nutrition or to a mixing of the characters of the two grafted individuals, but it is by no means easy in every case to separate one series from the other.

In considering variations due to nutrition, M. Daniel first gives an account of what takes place before looking for a further explanation. Four main divisions are made which it will be advisable to consider separately:—

§1. Changes in the size of Scion and Stock.—The dwarfing effects of grafting are, of course, noticeable in very many plants. Others attain their usual size, while this may even be exceeded in rare cases, where greater vigour is the result. To show that stunting is merely due to the difficulties put into the way of successful growth by the swollen tissues at the graft junction, plants were grafted on themselves, and so any chances of difference that might be looked for in two individual plants, even of the same race, were eliminated. Herbaceous plants, such as the Haricot Bean, which have but little secondary wood, when grafted upon themselves, reached only half the size of the control plants. The graft swelling was marked, the number of leaves small, and their colour pale like that of Haricots grown in a dry or badly-manured soil. In others, where secondary tissues play an important part, the normal size was attained. Cabbages grafted upon themselves grew as big as usual, and the graft swelling practically disappeared. This happened when stock and scion, or even the former alone, were vigorous, but when the stock was weak and the scion strong, the graft made but slow progress.

The results obtained by grafting plants of the same race, and of different races, species, or genera, are next described. The case of a free-growing Haricot Bean as scion, and a dwarf one as stock, has a special practical interest, for a well-branched plant of 6 or 7 feet in height, such as was obtained, is much more suitable for a garden than a straggling one two and a half times as big.

The age of the stock has a bearing-on the success of herbaceous grafting, for a Lettuce scion, say has no power to get nourishment from the "ripened" branch of a Salsafy; it cannot render the reserve food in the stock available, and so perishes. On the other hand, when the Salsafy branch in at the period of maximum absorption, a junction may be successfully made with the Lettuce.

In grafting woody plants upon themselves, the effects of the swollen union are at first the same as in herbaceous ones. These wear off in time. and so the prevalent idea may have arisen that the grafting of a plant upon itself never gives rise to varieties. It may be interesting to note the cases given where grafting causes abnormal rigour. This happens when a shoot of the Silver Lime is inserted into the common species. Grafts of a Service-tree on White-thorn in the public garden at Château Gontier were made about 1840. One specimen arising from a single scion has attained a diameter of 30 centimetres (10 inches), another from two scions, that of 40 cents. (15 inches) while ungrafted White-thorns have only a maximum diameter of 15 cents. (5 inches).

In the next place, reference is made to the grafting of species requiring, a flinty soil upon those which prefer a calcareous one, and vice versa. The growing of Pines, which object to chalky ground, on stocks of Pinus sylvestris, is alluded to, as well as M. Quintaa's idea of grafting the Chestnut upon the Oak, in order to give it more chance of warding off the ills that attack it when growing in a chalky soil. It is further pointed out that the various American Vines are not all suitable as stocks for French ones in every locality, and the one must be found by experiment which gives the best result with each soil and each scion.

In grafting plants that are variegated, the less the amount of chlorophyll the more difficult is it to gain success, and an attempt to use a completely etiolated branch always ends in failure. A reference to M. Daniels' first two figures will show what differences may be expected on the habit of a grafted tree, according as leading shoots growing away from the pull of gravity, or lateral branches which have lost the power, are used as scions. In the first case, the resulting Pear-tree, with erect branches, is a marked contrast to the other, where the boughs may rise, but little or ever droop.

Lastly, under this heading, the subject of grafting fruit branches on principal branches is broached, and this was done in the experiments, not only when the parts to be improved upon were woody branches as is customary in practice, but when these were young scions.

§2. Variations produced in the form, chemical constitution, and flavour of edible parts of grafted plants.— Experiments in grafting Cabbages are described at some length. In some kinds the leaves became more brittle; in others, the flavour was made less bitter and pleasanter to the taste. Generally speaking, vegetative parts showed a diminution in size, and in practice it would be necessary to secure an improvement in quality to counterbalance this.

Coming to reproductive parts, it was found that the grafting of the Cauliflower on the Cabbage cannot be used for the direct improvement of the former, as the flower-bearing branches lengthen. On the other hand, the round, yellow Tomato grafted on the large and early red variety produced fruit half as large again as those of the control plants. This is of special practical interest, as the yellow fruit is excellent but small. In a second case also the same result was obtained. Carrot fruits produced by the wild variety grafted on the cultivated one in the absence of any pollen of the latter, were almost double the size of those of the control plants. No general rule can be laid down with regard to herbaceous plants in this respect, but it is pointed out that the perfection of the graft unction, and the amount of sap reaching the scion, is as of much importance as in the case of woody plants.

§3. Variations on the time or manner of flowering.—The effects considered under this heading apply to the ordinary and not to the mixed graft. In annual plants, the time of flowering is, as a rule, retarded, but in an irregular way, and according to the plants made use of. In biennial plants, complications arise on account of the respective ages of scion and stock, when the latter is in its first year of growth, and the former in its second, the results already considered may be looked for to a greet extent. For instance, a Radish on a Brussels Sprout was retarded three weeks; and a Toad-flax on a Snapdragon a fortnight. When both components are in their first year, the grafted plant flowers at the usual time in the second, the effects of the healing period having worn off by then. Sometimes a like result is obtained when two plants are grafted in their second year. When a stock is tuberous, it is thickened by its scion in the great majority of cases. Exception must be made when the reserve food is inulin, which can only be used by a scion able to "digest" it. A Lettuce grafted on a Salsafy flowered much more rapidly than the control plants. Again, on grafting a biennial in its second year upon a perennial plant, an enormous amount of retardation of the time of flowering was obtained. Such in the exceptional case when the Salsafy is grafted on Scorzonera, for examples, grafted in the month of March did not flower until the following year. In retardation the healing process plays the important part; in acceleration the difference between the functional activities of the two plants produces the results, at least in the majority of cues.

When a stock has no reserve food that can be used by the scion, the latter uses its own supplies to form reproductive organs, and vegetative growth is stopped, as in the case of the Lettuce on Salsafy. Should the stock be without reserves, but able to absorb easily, as in the case of Cauliflower buds on a young Cabbage, after healing, which is a slow process, vegetative growth is continued while the stock is getting the materials from the soil which the scion needs, and flowering is delayed.

From the practical point of view these data may be of considerable value; by retarding the time of flowering until normal plants have passed that stage, hybridisation may be avoided, and by hurrying on the reproductive period ripened fruits may be obtained from plants that, as a rule, flower too late in the year for the seeds to be of any use.

In perennial woody plants flowering time is, normally, reached but slowly, and only when the tree is adult; the period of infertility varies with the plant. Grafting may lessen the purely vegetative period. With well chosen scions it is possible to obtain fruit in the first year. This must not be confounded, of course, with the hurrying on of the fruiting period in annual plants; these are induced to flower earlier in the year, the trees blossoming, with rare exceptions, at the ordinary season, but in an earlier year.

Grafting has also an effect on the arrangement of the flowers. These may, for instance, be crowded together; difficulties in flowering may also occur; petals may be crumpled, buds may become detached, and the blossoms may come to nothing, when those of control plants produce seed.

§4 Parasites in grafting.—The degree of resistance which plants offer to the attacks of parasites is of supreme importance in horticulture. Such resistance is increased with the vigour of the plant, and is lessened if the latter suffers or finds itself in unfavourable conditions, and grafting may have a marked bearing, temporarily or permanently, on the question.

In all grafts there is a time of provisional union, when the plants are most open to the attacks of parasites. When permanent union begins there is less danger it is true, but they are by no means safe. The kinds of parasites change, and they make themselves felt to a greater or less extent according to the perfection of the graft-junction, and the reciprocal conformity of functions in the two grafted plants. During the first period molluscs, such as slugs and snails, are most to be feared; their attacks are due to the lessened vitality caused by the operations in grafting, and not at all to any modification of tissues. Parasitic worms may enter through the wounds or soft tissues; woodlice and millipedes if in great numbers may endanger the success of grafts, and they attack by preference tubers and tuberous roots. There is little to fear from insects at this stage, though small beetles may do much damage. A species of Cheimatobius, for instance, lays its eggs in the buds on Apple scions, which should be carefully examined and the eggs removed, or some, if not all, the buds may be destroyed. Moulds do not develop unless there are sufficiently damp conditions, and the stock which has too much water suffers more, as a rule, than the scion which has too little. In the open air, except during heavy rains, moulds do not assume any importance, though when in had practice the graft and stock are placed under cover they may be of too frequent occurrence, seeing that neither plant is in the surrounding. which are suitable to it. When the provisional union is coming to an end, careful attention a necessary to ward off the ravages of parasites. Insects and millipede, come first in order of importance, then vegetable parasites, and lastly, molluscs, which in the previous consideration were most to be feared. Such a scheme is, of course, by no means absolute, and while woody plants suffer from the members of the first two categories, herbaceous grafts are most likely to be destroyed by those of the last. M. Daniel treats of this aspect in further detail, and sums up the facts by pointing out that the less perfect the connection between scion and stock the more likely are the attacks, saying, "Every badly-made or ill-assorted graft opens the door to parasites.”

NUTRITION.—Some sixty or seventy pages are devoted to theorising with regard to nutrition. The ideal behaviour of plants under imaginary and perfect conditions is dwelt upon, and what happens in actual cases the more easily worked out. It is stated as a principle that a wild plant in a given position never attains to its maximum size, because it never meet. with those perfect conditions in its surroundings which alone would allow it to bring out its full capacity of absorption and assimilation. This explains why, when man takes to altering the circumstances of growth, an in cultivation, a whole plant, or even a part of a plant, may reach a larger size.

Most principles of this kind, says M. Daniel, an now-a-days familiar enough to the physiologist, but not to practical men; and he does not think that they have been applied in any previous case to the phenomena of grafting. We cannot, however, follow M. Daniel through all his theoretical arguments, and must content ourselves with mentioning a few points that crop up incidentally. One method alluded to by means of which success in grafting was secured, is taken from the ordinary culture in this country of Vines, the foliage of which is kept in a house while the roots are outside. In a similar way in experiments the stock was kept in one temperature and the scion in another, so that the precocious plant was retarded, and the more slowly-growing one advanced.

Double grafting, as it might be expected, must double many of the disadvantages of the simple process, but an explanation of cases where the former operation is of importance is given. Plant A will not succeed on plant C owing to something objectionable to the stock in the elaborated sap of the scion. By the use of an intermediary, B, which is not affected, the difficulty is got over.

An interesting criticism of the expression that the French Vine when grafted upon the American "resists" the attack, of the Phylloxera, must not be passed over. It appears that such grafting is only successful in the presence of the pest. The insects remove a portion of the sap, which the more vigorous roots of the American stock can take up; if no parasites were present, there would be too large a supply for the needs of the French scion, which would suffer in consequence. Such bad results could, however, be overcome by allowing the shoots to grow from the stock, thus giving rise to a mixed graft.

A point not previously touched upon is the variation which may occur upon grafting, in the plants' power of resistance to cold. The more water present in the tissues, or, speaking practically, the less the branches are ripened, the greater the risk of damage by frost, and, in consequence, a stock which floods a scion with water is likely to cause its death in winter time should be avoided.

M. Daniel applies his theory to the culture of Apples. For the grower in a large way he advocates the use of trees on their own roots. These live and bear nearly five times as long as grafted ones. The gain in time of fruiting, he has already shown, is made up for by toss in the total amount of fruit produced, and there is the money to be reckoned which is spent on manure, labour, and the cost of replacing trees. As a rule, in the west, where trees are planted to the utmost extent that space will allow, the soil becomes exhausted in every way, and there is no room left for a plantation to be made which will come on and replace the older one. The fruit farms become great orchards of grafted trees, exposed to every malady that follows grafting, especially of a defective nature, and doomed to rapid decay. Under these circumstances, how can one replace the trees when the soil is exhausted as well?

The practice should be to plant moderately, leaving a sufficient apace between the trees, and preferably using trees on their own roots, only grafting such examples that produce inferior fruit, To the grower in a small way, who only thinks of quality, and uses or abuses grafting to get the best fruit as quickly as possible, this does not apply in the same degree, seeing that he can easily change his plants and his soil.

Double-grafting is strongly objected to, and it should, the author considers, be banished from the cultivation of the Apple. The process may double the profits of the nurseryman, as the trees quickly become marketable; and as a result of double-grafting they last for a much shorter time, and have consequently to be sooner replaced. The fruit-farmer's interests are the direct opposite of this last consideration, and he must use trees on their own stocks, or simply grafted. This is the only answer that M. Daniel can give to the question raised by the French Pomological Society in October, 1898, at the Congress of Mans.

GRAFT HYBRIDISATION

In the following chapter, variations caused by the mutual action of scion and stock are considered. In certain grafts of Alliaria officinalis on the green Cabbage, the characteristic odour of the former plant in sensibly modified, and can only be recognised after the leaves have been rubbed for some time, and the stock partakes of it somewhat. When the Cabbage is put on to the Turnip, the latter becomes sweeter than usual, and loses its characteristic flavour. A Haricot-Bean, cultivated for its seeds, and the pods of which are disagreeable to the taste, grown on the roots of a variety, the fruit, of which are eaten, produced some like these in taste, but of normal shape. These examples of chemical changes serve to prove, in M. Daniel's opinion, that the product of graft-hybrids is possible. The term, he says, is not very exact, but serves very well to show the origin of such plants.

Increased powers of resistance to cold, and to the attacks of parasites, in also claimed as being due to transmission of qualities from stock to scion.

Changes in the habit of growth next command attention. Among conical-headed Cabbages that had for stocks a variety that rejoiced in a round heart, were individuals that took this form and several intermediate shapes. Helianthus laetifolius, with a rhizome carrying a small, terminal bud at some 15 inches from the aerial stem, was grafted upon H. tuberosus, where the tubers normally remain close to the base of the stem. Though retaining their size, these bodies were produced at the ends of stalks some 10 inches in length. Somewhat the same result was obtained when an annual species of Sunflower was put on to the same stock.

Experiments on Potatos led to the conclusion that the grafting of two adult tubers is an impossibility, and that Trail's work cited by Darwin, had been incorrectly described. Plenty of grafts were made with young shoots, but the change in the colour of the flesh when one with white tubers was provided with roots that should have produced blue ones, was not necessarily a result of the process. White and streaked tubers occurred though in smaller numbers among the control plants. M Daniel is going to reverse the experiment to obtain more definite evidence.

When the woody Helianthus laetifolius was induced to unite with the Sunflower, the resulting plants with the latter as stock were flourishing after the control Sunflowers were dead. The stock had become very much more woody than usual, had lost its hairs and developed lenticels, so as to very much resemble the scion; this likeness was emphasised by a microscopical examination of the tissues.

Among trees, Mr. Daniel describes and gives photographs of the extraordinary branches sent out below the graft by the White-Thorn stock of a Medlar-tree at Bronvaux, near Metz. Some of these are furnished with thorns, their flowers are not solitary, but produce Medlars. There may even be on the shoots leaves of a character intermediate between the Medlar and White-thorn, while one starts with the typical foliage of the latter tree. This case is put down an a good instance of graft hybridisation, and M. Simon's opinion that the well-known Cytisus Adami is undoubtedly another example is given, and entirely endorsed by M. Daniel.

Perhaps the best results of this kind of hybridisation obtained during the experiments are met with in fruits. Striking figures of three forms from a single scion are given in the paper. These are from a shoot of the Egg-plant with long, violet fruits, grafted on a large, flat-fruited, red Tomato. One is the typical pyriform fruit to be expected; the second is egg-shaped, and small, such as would be produced by the white variety; and the third very much resembles the lobed and flattened fruit that would have been produced by the Tomato stock. Mention must also be made of the spines that appeared on the calyx of an Egg-plant normally without them, when grafted on a variety which possessed such structures.

With somewhat forcible arguments, the theory that graft hybrids are obscure hybrids of the sexual kind is dismissed. Solanaceous plants, for instance, have not been known to produce hybrids, and even if they did, the experiments show that grafting must favour sexual hybridisation; also a case is quoted from Darwin in which a Passion. flower on its own roots was not self fertile, but became so when grafted on another species. Herbaceous plants are more plastic than woody ones, and show more instances of graft hybrids when studied; but it is pointed out that these would no doubt furnish more evidence of their occurrence if the stocks were not trimmed carefully so as to leave no shoots in practice.

INARCHING.—Before proceeding to consider mixed grafts, allusion is made to inarching where the plants are not severed above or below the union, each still remaining in connection with its own roots. Such a process may have a practical value, for although it is of no use to increase the size of the plants, Cauliflowers from which seed is usually obtained with difficulty, readily fruited when treated in this way.

By mixed grafts a balance is able to be kept up between the functional activities of stock and scion, and success otherwise impossible obtained. Such a case is illustrated by the grafting of a deciduous plant upon an evergreen; here the stock expects the scion to work in winter, but having lost its leaves it cannot, and the graft fails. If some of the branches of the evergreen stock are allowed to persist, however, the difficulty is got over.

M. Daniel was enabled to maintain a Pear scion on a Crab-stock for a longer time than usual (l892-1898) by using the mixed graft, but being left to itself it became exhausted in the end.

Grafts were made during germination between the black Belgian Haricot (with three or four violet flowers in the inflorescence) and the large Soissons variety (with a score of yellowish-white blossoms in the head), the latter being the stock. One series had all the lateral-buds removed from the stock, and so consisted of simple grafts; in the second the shoots springing from the region of the cotyledons were allowed to grow, and mixed grafts resulted. As might be expected the evils of altered nutrition were apparent in the simple grafts, and these were stunted. Little apparent difference in growth was manifest however between the scion of the mixed series and the control plants. In one case, however, a graft produced a head of nine dirty-white flowers, streaked with violet.

It is not surprising that the chief differences should appear in reproductive organs to the formation of which both components of the mixed graft contribute material they have elaborated, and this is particularly the case when seeking seed from the graft, the stock has not been allowed to fruit, and vice versa. The practical value of mixed grafting for the production of hybrids without the intervention of sex need not be dwelt upon, but, at the same time its possibilities for lessening the undesirable variation due to nutrition alone must not be lost sight of.

INHERITANCE

The second part of the paper deals with the inheritance of the characters produced by grafting, and is based, naturally, upon experiments with herbaceous plants, seeing that the time necessary for observation upon successive generations of trees precludes such investigation upon the part of a single individual. As might be expected, the young of plants stunted owing to bad nutrition after grafting, were themselves degenerate. On the other hand, the large seeds produced by wild Carrots on cultivated stocks produced plants that approached to the latter in habit. A remarkable percentage of variations (the cotyledons varied in number from 3 to 1) was also noted, and several intermediate characters.

The seedlings from an Alliaria on Cabbage, recalled the appearance of the latter, and would have been set down as at least a new variety by a classificatory botanist. It has been found impossible to produce sexual hybrids between any two distinct species of Cruciferae by several workers, and the one recorded case is still contested. Hence the practical importance of a process which will bring about a variation in the first generation. Notable also are the Kohl Rabis obtained from the white variety grafted on the red; the "new" variety remained sound in the wet autumn of 1894, when many of the control plants rotted from the severe winter that followed, and while the former survived all the other members of the Cabbage tribe succumbed to the frost. Other instances are given, and the transmission of characters by vegetative reproduction of the stock shown to be partial with the Potato and Jerusalem Artichoke. From the seeds of the Haricot, Beans above described, where there were mixed grafts, the streaking of the flowers was not transmitted; but this is the only experiment at present made on such grafts.

M. Daniel concludes that the direct influence of the graft is not universal; that he has proved that actual variations, apart from the effects of nutrition, are at times induced; that these being acquired by the plant during its lifetime and transmitted to its offspring, Weissman's contention is not good so far as plant. are concerned. We will not here go into the question any further than to point out that what M. Daniel calls the progeny of a grafted plant—the true graft hybrid, in fact—may be looked upon as springing from two plants, when acquired characters in the special sense of the word might not come in.

Daniel bibliography