Plant Propagation (1916)
M. G. Kains
Professor of Horticulture, Pennsylvania State College

CHAPTER XII

DANIEL'S EXPERIMENTS AND CONCLUSIONS

207. Functional capacity of plants.—Daniel gives the following broad generalizations on the relationship between absorption, assimilation and transpiration in plants. By functional capacity is meant the processes involved in the absorption and utilization of crude materials by the plant. If the function of absorption or the total absorption from external surroundings be represented by CA, and the functional capacity of consumption or total consumption at the points where the sap is used up be represented by CV, then in a plant in complete equilibrium as regards its general nutrition CV= CA and CV/CA =1. If, however, aerial consumption is greater than the usual subterranean absorption, then the formula CV/CA > 1. This corresponds to growth in dry or poor soil. When absorption is greater than consumption, as in moist or rich soils, the formula becomes CV/CA < 1.

Conditions similar to these are sometimes brought about by grafting. The cicatrization of the grafted plants and the intercalcated tissue between stock and cion interfere with the condition of sap, modifying it both in quantity and quality. These modifications of the cion are equivalent to growth in a drier, poorer medium than the normal. In grafting it is also necessary to keep in mind the relative functional capacities of the two grafted plants. For example, if the functional capacity of consumption is greater in the cion than in the stock this condition becomes exaggerated by the scar of cicatrization when the two plants are grafted, and the graft either fails to take or makes a poor growth, corresponding to that in poor dry soil. The chance for making a successful graft in such a case is increased if the development of adventitious roots from the cion is encouraged, so that assimilation may correspond more closely to a normal absorption of the cion.

208. Graftage laws.—Daniel draws the following conclusions from certain of his experiments: 1, The relative affinity or difference of functional capacities between stock and cion at different periods of the symbiosis plays a very important role in the success, duration. and biology of all grafts. 2, Environment, weather, etc., particularly sudden changes of environment, have considerable reaction on the whole, a reaction greater than in normal plants. 3, Various irregularities like diseases result from faulty nutrition, due to badly chosen grafts.

209. Disease due to grafting.— A study of grafts between various species of Solanum (pepper, tomato, eggplant, etc.) has led Daniel to conclude that many of the physiological troubles of plants commonly considered as diseases are in reality due to the employment of antagonistic stocks and cions.

*L. Daniel, Rev. Gen. . Bot. 12, summarized in E. Sta. Record, Vol. 12, pp. 947-952.

210. Grafts are of two classes,* anatomical (grafts by approach) and physiological (true) the latter divided into two groups, ordinary and mixed. In the ordinary the stock is deprived entirely of its assimilating apparatus (the green parts) and the cion of its absorbing parts (the roots). In the mixed graft proper the stock may preserve part or all of its assimilating apparatus and the cion part or all of its absorbing apparatus. In grafting by approach success is attained when the two plants grow together in an enduring manner so that if separated a wound is formed. The graft proper is said to succeed when, after having lived a certain time on the stock, the cion produces fertile seeds.

The conditions of success of grafts are of two kinds, extrinsic (conditions independent of the nature of the plant—as soil, temperature, etc.) and intrinsic (conditions dependent upon the peculiar nature of the plants grafted, as method of cicatrization, analogy and botanical relations. The extrinsic conditions necessary to observe by approach are: 1, A temperature sufficient to produce primary tissue; 2, prevention of all conditions which cause rotting or drying of the cicatrized meristem (primary tissue) ; and 3, maintenance of adherence of the wounds by the aid of ligatures susceptible of being loosened progressively with the growth of the plant.

211. Cicatrization.—Plants cicatrize their wounds either by simple drying of the cut tissues and neighboring cells or by regeneration of tissues by the aid of the primary tissue. All methods have failed with the monocotyledons and cryptogams experimented upon. Hence Daniel concludes that grafting by approach is impossible with plants that cicatrize their wounds by desiccation of the wounded cells and neighboring tissues; i. e., are incapable of regenerating tissue.

In order to learn whether only plants with cambium may be grafted, as generally believed, Daniel worked with many monocots and cryptogams and secured a perfect cicatrization by the tongue graft with gladiolus, day lily (Funkia cordata), Caladium, white lily (Globba coccinea) and several others, even with one of the club mosses (Selaginella arborea). The success of these grafts shows that grafting by approach is possible with certain monocots and that the presence of the cambium layer is not always necessary to the success of all grafts by approach.

212. Analogy.—The difference in hardness and the histological nature of woods may not be an obstacle to anatomical union. A natural, distinct cicatrization occurred between grafted oak and beech, and between fir and linden, oak and ash united by their stems and oak and walnut by their roots. Rose and grape have also been united. Nevertheless, the graft by approach does not always succeed between plants so different. Daniel tried in vain to graft horse chestnut on common chestnut.

Accumulation of reserve material in vegetative plant parts has no special importance in grafting, as proved by grafting turnip and cabbage, kale and kohl rabi, Brussels sprouts and kohl rabi, and kohl rabi and cauliflower. [These plants, though very different in form as cultivated, are all, except turnip, varieties of one species, Brassica oleracea—-M. G. K]. Even grafting by approach between roots of lettuce and aged salsify succeeded, though the inulin of the salsify did not circulate in the lettuce cells. If the cell contents of one of the plants approached are toxic for the other, the graft fails.

If a large and a small variety are grafted on each other the larger will develop to the detriment of the smaller, which will remain nearly dwarf. Plants of different forms, like kale and cauliflower [see bracket note above], may make good unions. Plants in active growth may be grafted by approach on plants at rest; e. g., seedling cabbage several weeks old was grafted in spring with perfect success on a turnip whose root was fully formed, Grafting by approach succeeds between annuals, biennials, and perennials; also between biennials and perennials. The fact that fir and linden, and Aralia Spinosa and A. Sieboldii were grafted by approach shows deciduous and evergreen plants may be intergrafted.

213. Extrinsic conditions.—With grafts proper all extrinsic conditions are present, but in cutting off the top of the stock and suppressing the absorbing apparatus of the cion, there is danger of the death of both plants. Another fundamental extrinsic condition of success must be maintenance of life in the two plants till success is complete.. The cion is sometimes preserved by being placed in water as soon as made to prevent its drying out, keep the surface clean, and prevent the formation of sugar or other material on the cut surfaces. This might interfere with the free passage of sap from stock to cion.

In order that the cion may grow its turgescence must be reestablished. This is secured by the imbibition of the crude sap of the stock by the cells of the cion, and occurs more quickly according as the crude sap is presented in considerable quantity, but also more quickly if the initial turgescence of the tissues of the cion has not been diminished during the preparation of the graft and its being put in place. This explains why it is necessary to operate quickly and why cuttings are often made under water where the cion preserves much of its turgescence and produces good results. It also accounts for the good effects of wax-like material used in open air grafting; the utility of the collar graft, because the osmotic force is strongest at this level; and the importance of the time of day in operating, because the osmotic force varies, being strongest in the evening (hence the greater success then). The re-establishment of turgescence in the graft is considered fundamental to success, therefore it is impossible to graft parts incapable of retaining turgescence or which do not possess it.

214. Intrinsic conditions.—In the graft proper plants incapable of regenerating their tissues cannot be grafted. In gladiolus and Funkia cases cited above, the anatomical cicatrization was effected by the parenchymatic tissues. No liber or fibro-vascular structure was observed to form between cion and stock. Thus the transport of sap was hindered, and sooner or later both parts died. By utilizing the aerial roots of some monocots to supplement the absorption of the cion, success was attained with several plants. This shows that failure of grafts with monocots capable of regenerating their tissues is due to insufficient vascular communication, since it becomes possible when a complimentary apparatus is supplied.

Plants with active cambium layers, which may be inarched, cannot always be grafted by the graft proper, since the common European bean (Fabia) and the kidney bean, which graft easily by approach, have always failed when grafted by the graft proper, no matter what precautions were taken.

Differences in wood and bark are not obstacles to success in the graft proper. Thus there is a great difference in the thickness and strength of safflower and annual sunflower; between sunflower and Jerusalem artichoke; young cabbage and root of turnip; root of cultivated carrot and that of fennel; nevertheless, these [pairs of] plants united perfectly. These same facts were observed with trees; the graft succeeded between chestnut and oak, pear and hawthorn, quince and hawthorn, in spite of marked differences in the barks. From these and other grafts it is concluded that hardness, density, and elasticity of wood are secondary in the success and duration of grafts, but it is not the same with conduction.

When the differences of sap conductions are too great, grafts will not succeed; e. g., lilac and ash, cherry and almond, Cotoneaster and chestnut, which grow the first year, then die without fructifying. The duration of the graft is then very variable and depends for its value on differences in conduction between cion and stock. Thus pear grafted on quince endures for a shorter period than pear on pear seedling [the Yeoman's dwarf pear orchard at Walworth, New York, bore profitable crops for over 50 years! M. G. K.] When the differences of conduction are too great between plants, the mixed graft is sometimes used successfully where the ordinary graft fails. By using it Daniel united Vernonia praealta and Xanthium macrocarpum, which failed by ordinary grafting.

Daniel has succeeded in grafting plants whose cell contents presented very marked differences; e. g., Chicoreaceae and Euphorbiaceae, which have different latex contents. Previously it had been held that plants with milky juice could not be grafted.

Grafts were made to determine what influence reserve material in plants may have on grafting. The easy grafts on roots of carrot and parsnip show that the presence of reserve material is no obstacle to success. Those of tomato on potato, annual sunflower on Jerusalem artichoke, etc., show that the formation of tubers on the stock takes place even when the cion is incapable of producing tubers itself. In grafting in September a young cabbage on a purple-topped turnip, which would have begun to thicken its root, in October, the thickening came in the April following, when the cion became plethoric. It is, then, the cion which by its mode of nutrition commands the function of reserve material in the stock.

FIG. 141—SWELLINGS DUE TO GRAFTAGE
1. Large growing apple on small growing stock. 2. Swelling of tissues at point of union. 3. Pear on small growing stock.

The inverse graft of plants susceptible of forming tubers on a plant which does not yield tubers may be realized. Daniel succeeded in grafting Helianthus loetiflorus, a species with an enlarged rhizome, on H. Annuus, an annual species which does not form tubers. The cion grown entirely above the soil was unable to form tubers. The reserves passed into another form in the stock, which took a development altogether abnormal and became very ligneous. Potato grafted on eggplant and on tomato has been observed to form aerial tubers and thus store up its reserve material.

215. Analogy in habitat seems to be a more or less important factor. Thus Phlox decussata, which grows in humid soils, has not been successfully grafted by Daniel on P. subulata, which grows on dry soils; though parsley, which prefers a dry soil, succeeds when grafted with Sison ammonium, which prefers humid soil. In the case of trees, pears are grafted on quinces in rich soil and on pear seedlings in poor soil, etc. Different soils, then, are not the most serious obstacles to success in grafting, but they seem to have more or less marked influence on the duration of the graft.

If a dormant ligneous cion is grafted on an active ligneous stock, success follows, but does not follow if conditions are reversed. With herbaceous plants, an active cion may be grafted on a dormant stock and succeed. When cion and stock do not come into activity about the same time, the graft may succeed, but its duration will be shortened.

In order to study the limits of the possibility of grafting, experiments were made with Rosaceae, Umbelliferae. Leguminosae, Cruciferae, Solanaceae and Compositae. With Rosaceae, Leguminosae and Cruciferae the limit of grafting seems to be confined to genera of the same tribe. With Solanaceae and Umbelliferae grafts were successfully made between different tribes. With Compositae the limit seems to be the sub-family.

* Extended summary in Experiment Station Record, Vol. 5, p. 1089.

216. Herbaceous grafting has been successfully practiced experimentally by Daniel,* with pea on bean, cabbage on kohl rabi, turnip, stock and other related plants, fennel on wild carrot, carrot on parsnip and vice versa celery on parsnip, winter lettuce on wild prickly lettuce, spring lettuce on salsify, salsify on scorzonera, toadflax on snapdragon, almond, peach and prune on cherry.

This experimenter also found: that grapes will unite between genera of the same order; 2, hollow-stemmed annuals unite, while the pith is functional; 3, with trees the union is easily made and the swelling at the point of union is reduced to a minimum; 4. root grafting of herbaceous plants is most successful; 5, duration of grafted plants is more or less modified by the graft, a, annuals on biennials or perennials continue to be annuals, i. e., at the end of the growing season they die and cause the partial or total death of the stock; b, biennial grafts with rare exceptions remain biennial on both biennial and perennial stocks and induce the death of the stocks; c, perennial grafts on annual or biennial stocks die with the stocks, but may be used as grafts on perennial stocks prior to this event. 6, herbaceous grafts are less resistant to cold than are mature wood grafts; 7, time of flowering is slightly retarded among annual grafts, and at least for the first year among biennials and perennials; 8, stock and cion are influenced reciprocally—sometimes cion controls stock, sometimes vice versa, and sometimes both classes of cases may be found in the one kind of graft but with different specimens. 9, Cultivated varieties grafted on wild ones generally show deteriorated quality in the fruit; 10, seed produced in such cases (9, just mentioned), some seedlings revert to the wild type and largely lose value as food plants, hence it is concluded that inferior stocks should not be selected for grafting when seed is to be saved for planting; 11, reserve food of a stock is rarely utilized by the graft of a plant of another family.

Circulation of liquids in grafted plants presents two extremes to consider: 1, cases where water absorbed by the stock passes in small quantity into the coin; and 2, where it is abundant. In the former (the commoner) the cion makes less vigorous development but flowers and fruits more abundantly than in Case 2 where the vessels are larger, the sprouts more vigorous and the flowers and fruits less abundant, as in the case of a tree too well nourished. At first these phenomena are physical, but later chemical changes modify the cell contents as shown by starch or sugar formation under the influence of the graft in certain cases.

217. Reciprocal influence of stock and cion.—Daniel has demonstrated both direct and indirect influences of stock on cion in grafted plants. From his experiments he deduces that variations in the graft may be due to changes in nutrition or may he specific: that is, they may appear in particular characters of stock and cion more or less independently of environment. The effects of grafting on the general nutrition may be shown in four ways.

1. The size of the vegetative organs of stock and cion may be modified. If a herbaceous plant is grafted on itself, the general nutrition is interrupted in inverse proportion to the activity of the cambium layer at the time the operation is performed. The same principle applies in the case of grafts between different plants of the same variety.

In the case of grafts between plants of different varieties, species or genera, the reciprocal reactions are much more complicated by the imperfect functional adaptations. In herbaceous grafts the callus has the same effect on the cion as would placing it in arid soil it is dwarfed. In determining what species of the same order may be successfully grafted, similarity of habitat is of more importance than relationship. In the case of ligneous plants, geotropism (219) is a cause of variation, although this has generally been denied heretofore. It is also shown that a branch that has lost its negative geotropism does not always regain it if grafted on the main axis of the stock, at least in the case of the pear.

2. The flavor of the edible parts, size, chemical composition or season of development, may be modified. If the union is perfect, grafting in general produces a change of flavor in the edible parts of vegetative organs, either in the nature of improvement or deterioration in quality. There is almost always a reduction in size of the part which sometimes fails entirely to develop in edible form. For the operation to have practical interest, the, diminution in size must he compensated by increase in quality. When the edible parts belong to the reproductive organs, grafting herbaceous plants may or may not cause the enlargement of the pericarp of fleshy fruits or of the seeds in dry fruits. There is no known principle of general application. The flavor of the fruit depends mainly on the completeness of the union and the quantity of sap the cion receives. This principle applies to ligneous and herbaceous plants alike.

3. The development of the reproductive organs of the cion may be accelerated or retarded. The flowering season of the cion may he affected very differently, according as the plant is annual, biennial or perennial, according to the age and nature of the cion, and according to the kind of graft employed. Grafting may induce variation in the arrangement of flowers, in the season of opening or falling of the petals, or in their color.

4. The relative resistance of stock and cion to parasites and other injurious organisms or substances may be modified. The principal parasites that attack grafts before the union is complete are molluscs, worms, sowbugs, insects and molds. These parasites may affect stock and cion differently. The parasites that attack grafted plants after the union is complete, are in the order of the extent of their ravages, insects, myriapods, fungi, and other vegetative parasites, and molluscs. The more imperfect the union of stock and cion, the more serious are the attacks; so much so that their extent and severity may be said to be a criterion of the degree of perfection of the symbiosis.

218. Reasons for the above effects.—The theory by which Daniel seeks to explain these facts is in brief as follows: The nutrition of stock and cion is modified by two causes which may act in the same or contrary directions. These are (1) the callus consequent on the operation, and (2) the difference between the peculiar functional capacities of stock and cion, such as differences in structure, special diastases, differences in composition of the crude or the elaborated saps, etc. The phenomena produced are dependent, not only on the nature of the plant, but intimately so on environment.

From this theory certain conditions of success in grafting may be deduced. The protoplasm of stock and cion must not, as a result of the operation, be modified beyond that definite point at which poisoning sets in or at which the essential properties of the living substance, as nutrition and motility, are destroyed. Destruction of the protoplasm may result from either of two causes: (1) action of plastic or waste products brought together suddenly, causing immediate poisoning or gradually causing slow poisoning. These products may give rise through mutual reactions to other injurious products. (2) Deficiency or excess of water in stock or cion consequent upon grafting.

Daniel demonstrates the insufficiency of the hypothesis of relationship and that of similarity in composition of elaborated saps to account for the success of a graft or to explain its variations. He reports a large number of experiments, each illustrating a different variation, produced directly by a mutual reaction of stock and cion. Specific variations differ much in degrees according to the nature of the plant and even according to the part of the cion. The principle applies alike to herbaceous and woody plants. Specific variations result in a more or less complete blending of the characters of stock and cion; or more strictly, these characters appear side by side but separate and distinct.

219. Geotropism, the tendency of plant parts to grow downward toward the center of the earth, as in ordinary roots. Negative geotropism (apogeotropism) is the growing away from the earth, as in ordinary stems.

220. Transmission of grafted characters by seed.—From certain experiments Daniel concludes that variations due to nutrition are in some cases transmitted by seed collected from the cion, even when no morphological changes are apparent in the cion itself. Such cases show that the immediate influence of stock on cion may be less than the indirect influence of the offspring of the cion. Seed grafts of wild carrot on the cultivated half-long red variety showed clearly such a mixture of the characters of stock and cion that the resulting plants might be considered true crosses or graft hybrids produced by the influence of stock on embryo. These and similar experiments show also that by grafting a wild and a cultivated plant the former may be made to acquire definite qualities which can be improved by selection. Experiments showed also that these variations, which the experimenter classes as specific, are at least in certain cases transmitted by the seed.

Daniel also concludes that grafts may influence the somatoplasm (223), though not always. In many plants the effect is often very slight, especially in woody plants in which the ligneous framework gives to the plant a much more fixed form than herbaceous plants possess. When this influence exists, it most often affects characters of little taxonomic importance, as height, vigor, etc., and then its influence is similar to that of environment; but it may sometimes affect the essential characters of varieties or species, such as external form, structure, etc., which become more or less blended into graft hybrids (228) or may disappear, giving place to new characters. Not only may the influence of the graft on the somatoplasm show itself directly in the grafted plants themselves, but it may produce an indirect reaction either parallel or not parallel to the direct reaction, and new characters may develop in the offspring, proving that, contrary to Weismann's theory, acquired characters can be transmitted in the vegetable kingdom.

From his theoretical considerations, Daniel deduces certain practical conclusions. When grafting does not modify the peculiar characters of a variety, but merely produces certain slight variations of nutrition, it may be employed to perpetuate varieties, races, or accidental forms of perennial plants; but if the influence of the graft on the somatoplasm is very marked and proves to be specific (which experiment alone can determine), it may be applied to the creation of new varieties. Here a new field of operation is opened up to seedsmen.

There are numerous practical applications of the reciprocal effect of stock and cion, such as increase in size of fruits, improvement of flavor of fruits and certain vegetables, production of new varieties in which color of flower, form of fruit, or vegetative organs are modified. The effects of grafting are more marked in herbaceous than in Woody plants, and also more marked in the offspring of the grafted plant than in the plant itself. Grafting, as a means of retaining variations acquired under culture, is useful only in the case of trees, and difference between seedling fruit trees and varieties producing them (199) may be explained in part by the effect of grafting on the progeny of the grafted plants.

Grafting which produces a variation in the seed may be used to produce new varieties. Since this variation can frequently be directed in a given way, it is possible almost to a certainty by repeated grafting to impart definite characteristics of flavor, form, color, etc., to plants which vary readily under culture. In other cases grafting may produce variations which, though hard to obtain, after once appearing, may be directed definitely.

221. Asexual hybridization—Formerly it was believed that grafted cions lost none of their own characteristics and acquired no new ones from the stocks on which they were grafted, but the experiments of Daniel and other investigators indicate that these views must be modified. For Daniel has proved that hybrids produced by grafting can be fixed and propagated true to kind, but he draws the conclusion that asexual hybridization is neither constant, regular nor very frequent. In its results it is somewhat similar to cross pollination, but has a wider application, and the resulting forms are less constant in character.

Grafting is not always a certain means of perpetuating variations, although it generally is. In itself it may occasion variation, which in turn may be fixed by grafting. In order to produce a given variation by grafting or to add to a plant a character it lacks, it is necessary to graft it on another plant which is superior to it in the quality sought. In grafting hybrid vines to secure a desired character, it is necessary to graft together two vines having common blood in such proportions that the sum of the blood of the characters desired shall be greater than the blood of any other strain in the graft. Modification in vines as regards eradication of the foxy flavor, increase in size of berry, resistance to exterior agents, etc., can thus be obtained.

The problem of the French grape industry, Daniel declares, is to unite American root resistance (to phylloxera) to the French quality fruit. He says this will probably be (lone, not by sexual hybridization alone, but by rational combination with asexual hybridization by grafting, and then the preservation of the variation thus secured by budding.

Asexual hybridization, according to him, sometimes operates directly upon the grafted plants, sometimes indirectly upon the descendants; sometimes it affects external characters; sometimes it causes a disjunction of previously blended characters; sometimes heredity and persistence are complete; sometimes partial or lacking; often expected results can he secured. But the most important practical point is that in many cases grafting has served to insure systematic improvements of plants. Emphasis must be laid on his remark that when a plant is to be improved in a certain respect, it must be grafted on a stock superior in this respect.

222. Germplasm, the continuously living substance of an organism. It is capable of reproducing both itself and the somatoplasm, or body tissue, in giving rise to new individuals. It is the Substance, or Essence, or Life which is neither formed afresh, generation after generation, nor created or developed when sexual maturity is reached, but is present all the time as the potentiality of the individual, before birth and after death, as well as during that period we term "life" between these two events. The somatoplasm, on the other hand, has no such power. It can produce only its kind—the ephemeral, the perishable body or husk, which sooner or later completes its life cycle, dies and disintegrates. The germplasm, barring accident, is in a sense immortal.

223. Somatoplasm, the body tissues as a whole. See Germplasm (222).

Daniel bibliography