Various problems are raised by the fact that fruit trees are of necessity composed of two individuals growing together as one. A review of the literature shows that not only may the rootstock affect the cion, but also that the cion and an intermediate stem-piece may have an important influence upon development of both the root and the entire plant. An important question has been introduced into a consideration of the use of vegetatively propagated rootstocks as improved stocks for fruit trees by the indication that the cion and an intermediate stem-piece may in some instances dominate root development to such a degree as to negative the effect of the rootstock upon the cion.
Results are reported dealing with 1,800 French Crab piece-roots grafted to 14 varieties and clons of apple in 43 combinations of single- and double-working. Photographs from the first season's growth of both single- and double-worked grafts show a disposition for the top cion to dominate the root system of the rootstock, whether an intermediate stem-piece of some other variety has been introduced or not. Vigorous cion development has resulted in vigorous root development. The least vigorous grafts have roots which develop at an acute angle with the vertical. Photographs taken at the completion of three growing seasons show that in some instances the top cion has had an appreciable influence upon the growth and character of the rootstock and of the grafted plant, while in other instances the intermediate stem-piece has had an effect upon the entire plant.
Two major factors have seemed to dominate the situation, namely, the nature of the top cion and the compatibility of the union. In combinations in which the graft unions appear smooth and congenial so that the union in itself is not a limiting factor, it has been the top cion which has dominated the shape and character of the rootstock, and not the intermediate stem-piece. The effect of cion foliage is considered in this connection.
While the intermediate stem-piece has seemed not in itself to have affected the root directly, yet it has in some instances affected it indirectly thru an influence upon the entire plant, such as dwarfing. Where a dwarf stock has been used as either rootstock, intermediate stem-piece, or top cion, the effect has been to dwarf the entire plant. The nature and degree of union is discussed from the anatomical standpoint as bearing upon this question.
In the consideration of the "stem" effect in double-worked trees emphasis is placed upon the region of contact between combining tissues, the "union," rather than the "stem" itself, as the more important consideration.
The question is no longer which one part dominates the entire plant, since all have been shown to have marked influence at one time or another. The rootstock may be the limiting factor one time, the union another, and the top cion another. The question is rather one of relation and degree between the consorting parts. The combination and intergrading of these factors determine the "build-up" of a fruit tree.
New York State
Agricultural Experiment Station, 1933
TECHNICAL BULLETIN No. 218
INFLUENCE OF THE CION AND OF AN INTERMEDIATE STEM-PIECE UPON THE CHARACTER AND DEVELOPMENT OF ROOTS OF YOUNG APPLE TREES
H. B. TUKEY, Chief in Research (Pomology), AND KARL D. BRASE, Assistant in Research (Pomology)
Fruit trees, unlike cereal and vegetable crops, are composed of two individuals growing together as one. Cereal and grain crops are propagated from seed, so that the root and the top are from the same original source and are, in fact, one and the same plant. With fruit trees, on the other hand, this is not true. Instead, a rootstock is propagated from seed or by some vegetative means, as from layers or cuttings, and onto this rootstock is budded or grafted the variety desired for the top of the plant. The result is a plant composed of two different individuals growing together, the one dependent upon the other, each for particular functions necessary for survival.
|1Refers to Literature Cited, page 46.|
It may be asked why horticulturists must resort to this method of propagation. First of all, fruit trees will not come true from seed. Altho seed from certain varieties, such as Fameuse, Delicious, and McIntosh, results in more uniform seedlings than that of some other varieties, such as Baldwin, Rhode Island Greening, and Gravenstein (65),1 yet the variation in either case is large.
In the second place, fruit trees are notoriously difficult to propagate by vegetative means. From the theoretical viewpoint, propagation of fruit trees by layering or from stem or root cuttings would be desirable. Unfortunately, no commercially successful (3, 10, 30, 22, 33, 34, 36, 57, 65, 66, 69, 70, 72, 75) method of vegetative propagation has yet been devised, and in addition, it is not always true that a variety will perform better upon its own roots than upon roots of some other plant (57).
Accordingly, for rootstocks, the practice in nursery circles is to resort to materials which can be quickly and easily propagated and which will give a fair degree of uniformity for commercial orchard purposes. Two methods are in general use, namely, root grafting and budding.
AMERICAN METHODS OF PROPAGATION
The method of root grafting involves the whip grafting of a cion 3 or 4 inches in length upon a piece of root 6 to 8 inches in length during the dormant season and the planting of the graft out of doors the following spring. The type seedling desired for this method is one with a long straight root, so that more than one graft can he secured from each seedling. In the sections where this method is practiced, notably the central west, the graft will grow into a salable tree the first year it is planted.
Contrary to much opinion, especially in European circles, bench grafting is not the principal method in use in America. In eastern sections of America, for example, grafting is not used at all, for the reason that there the grafts do not make sufficiently sizeable trees in one year and also are more likely to develop hairy roots and crown gall, rendering them unfit for sale. Accordingly, the accepted method is budding (Fig. 1) in which a seedling rootstock is planted in the nursery row to grow for 1 year and upon which a bud of the desired variety is inserted on the stem about 2 inches above the ground. The second season after planting. the top of the seedling stock is cut off just above the bud, the bud grows rapidly, and a 1-year tree 5 to 6 feet high is produced in one season's growth. Until relatively recent times this method has been the one most used in America and is still used almost exclusively in eastern sections. The majority of American orchard trees, therefore, have been raised by budding rather than by piece-root grafting. The type seedling desired by nurserymen for budding purposes is one with branch roots in contrast to the straight-root seedling desired for piece-root grafting.
PROBLEMS RAISED BY PROPAGATION METHODS
It can he seen at once that various problems are presented by this situation. First of all, what effect may the rootstocks have upon the plant? If the rootstocks are raised from seed, will they not be variable and will not this variability show itself in the resulting grafted or budded tree (2, 20, 25. 54) ? Or if, using Knight's argument (30) that, "It is much easier to improve the propagation of a variety of good performance, than to improve the performance of a less desirable variety, which happens to be easily propagated', it is attempted to raise clons of uniform rootstocks, what effect may they have upon the orchard tree? What of hardiness of both stock and cion, disease and insect resistance of both stock and cion, and soil adaptation? What of the congeniality of stock and cion, strength of union, and longevity of the orchard plant? All of these problems immediately present themselves.
|FIG. l.—STEPS IN THE PRODUCTION OF AN APPLE TREE BY BUDDING—THE MORE COMMON AMERICAN METHOD. 1, seedling understock grown from seed in one year (1930), to be lined out in the nursery the following spring (1931), for budding (1931) 2, budded seedling understock, showing bud in place (1931) ; 3, yearling whip (1932) ; 4, two-year tree (1933).|
REFERENCES FROM THE LITERATURE BEARING ON THE PROBLEM
EARLY AMERICAN EXPERIENCES WITH EFFECT OF CION UPON ROOTS OF FRUIT TREES
That the stock may have an effect upon the behavior of the cion has been appreciated from time immemorial and need not be discussed at great length here. On the other hand, the effect which the cion may have in its turn upon the stock has been less well understood, altho the fact is not new. Between 1857 and 1867, French, English, and American literature abounded with instances of reciprocal stock and cion influence. In fact, there was so much interest in the problem that in 1860 the Federation of Sociétiés Horticoles de Belgique offered a prize for the best memoir relative to the subject.
|Prince (1832) wrote: "I have now to state to you what I have never met with in
any author, that the graft has an influence on the stock and root of the tree."
American horticulture is relatively young, yet as early as 1846, Goodale (12) called attention to the influence of Domestica plums upon the color and size of roots of the Canada plum when budded upon the latter as a rootstock. Again, in 1847, he (13) pointed out that apples both budded and grafted upon seedling rootstocks show an effect upon the roots. Harwell (18) in 1850 indicated that in combinations between the cherry and Chickasaw plum, the cion determined the time of bud start in the spring. Subsequently interest waned, yet the reciprocal effect of stock and cion was appreciated nearly 100 years ago in America.
The topic was again revived between 1870 and 1880. Hoopes (28) recorded in 1873 this interesting note,
"The very moment that an inserted bud or graft commences to granulate and then unite, that moment the two parts of the embryo tree struggle, as it were, for the mastery. That is, certain idiosyncrasies inherent either in the branches of the one, or the roots of the other, will form a leading feature in the mature plant. Abundant proof of this is afforded by examining the roots of nursery-grown apple trees, whether budded or grafted. Take for instance some well-known variety, as the Bellefleur, and the roots will be found uniformly long, slender, and very fibrous; other kinds will prove exactly the opposite." Talbot (63) in 1879 pointed out that upright-growing varieties cause the roots of seedlings to grow downward. He also makes the interesting comment that, "It is the sap elaborated in the leaves of the stock passing into the graft which may be responsible for alterations in the character of the fruit borne by the graft".
The main feature of the discussion in the late '70s and early '80s was, however, whether the changes brought about by cion upon stock, and the reverse, were permanent in nature. The Massachusetts Horticultural Society records a lengthy controversy (63) in which some of the leaders in American horticulture expressed their views. That there was a reciprocal effect of stock and cion seems to have been one of the premises of the debate.
It is certainly true that American horticulturists (16) and nurserymen, in particular, have recognized for a long time that the cion may have an effect upon the root development of nursery trees. Nursery foremen, for example, have had the practice of putting more men in the gang or putting an extra team of horses on the digger when the Oldenburg apple is dug, whereas in the case of the Red Astrachan they use fewer horses and fewer men. The fact has been such common knowledge that it has been accepted and passed with little comment.
MORE RECENT EXPERIENCES WITH EFFECT OF' CION UPON ROOTS OF FRUIT TREES
Nevertheless, Hedrick (24) pointed out in 1915 that, "The cion in its turn has a decided influence on the stock. We know certainly, for example, that the form of the roots is much changed by the cion. Thus in starting apples in a nursery, we bud on seedlings which unbudded would have root systems much the same, but at digging time the roots of the different varieties are as diverse as the varieties themselves; Red Astrachan, for instance, has an exceedingly fibrous root system with few tap roots while grown on either side of the Red Astrachan row, Oldenburg and Fameuse are almost destitute of root fibers, and have instead deep tap-roots with two or three prongs."
Shaw seems to have been the first to examine the root systems of nursery trees critically with particular reference to the effect of the cion variety upon their development. He (55) showed in 1915 that strong-growing varieties in the nursery have strong-growing roots that the branch angle is correlated with the root angle, and that the size and stoutness of the branches is correlated with the size and stoutness of the roots. He cites as examples the Rhode Island Greening, which has a flat top and spreading roots, and the Oldenburg, which has two or three stout ascending branches and similar deep roots. He says that nurserymen recognize a correlation between tops and roots in ornamentals and shrubs and concludes that, "The cion governs the size, and extent, and form and disposition of the root system."
Swarbrick (58), upon his return to England from a visit to America, remarked upon the ability of American nurserymen to locate fruit trees in bins corded in tiers, roots outward, by the root habit, lie also indicated that employees recognize certain varieties as easy to dig and others as less easy. Maney (37) has shown that the Sharon apple produces strong heavy roots on French Paradise stock which is normally fibrous.
More recently, Swarbrick and Roberts (62) and Roberts (46, 47, 48) have placed on record the observations of nurserymen and practical horticulturists in regard to the effect of the cion variety upon root development of seedling rootstocks, and present experimental data to show this effect with apple trees.
EXPERIENCES WITH OTHER PLANTS
It will not be necessary to review European experiences in detail except as they find immediate application to the present problem, inasmuch as Gardner, Bradford, and Hooker (11) have brought together the most important literature upon this subject, while Swarbrick (60) has reviewed the older literature and Hatton (20. 21) has given a discussion of more recent literature. Yet it should be indicated that literature records ways in which a cion may influence a rootstock other than in the quantity of roots and the direction and form of roots.
The longevity of the root may also be affected. Sahut (53) records various cases of evergreen cions upon deciduous rootstocks in which the stock remained long-lived. Hofmann (27) has shown more recently that when the navy bean (Phaseolus vulgaris) is grafted with the lima bean (P. lunatis), the length of life of the former may be materially lengthened, whereas the reciprocal graft may shorten the length of life of the lima bean used as a rootstock.
Hardiness is also involved. Vard (71), following the severe winter of 1890-91 in France, showed that hardy stocks top-worked with cions of tender varieties, such as the tea and Bourbon roses, were killed; and in no instance were such top-worked stocks as hardy as he ungrafted hardy types. Likewise, Milliken, et al. (39), refer to the definite influence of the tops upon the stock in grafts of citrus varieties. Not only were the tender lemon tops killed, but the injury from freezing extended 3 to 4 inches down the normally hardy stock.
Pomelo seedlings when grafted with cions of the tender varieties were likewise injured, while the ungrafted stocks were scarcely touched. Filewicz (9A) showed a similar situation with apples.
Variegation has been listed (35) on numerous occasions as one of the effects of the cion upon the stock, but these instances would seem to involve inoculation of the stock with disease, such as mosaic.
Color, flavor, and shape of fruit have been mentioned (26, 64) as affected by the stock and the cion, tho of limited degree and possibly correlated with foliage development, as will be shown later.
Chemical composition has been altered. Meyer and Schmidt (38) have shown that a soluble alkaloid, such as nicotine, will pass freely from a tobacco cion to a potato rootstock. On the other hand, Guignard (15) says that glucosides do not pass from one graft symbiont to the other, agreeing in this respect with Hofmann (27) who shows that anthocyanin will not pass from the cion of a red strain of Refugee bean to a stock free from anthocyanin. These instances show the selective absorption of stock and cion which is further corroborated by Roach (44) who finds molybdenum present in the roots of apple rootstock Malling IX but not of Malling XII.
STOCK AND CION INFLUENCE IN RELATION TO VEGETATIVELY PROPAGATED STOCKS
Observation that the cion and stem portion dominate the roots.—An entirely new and important aspect to the problem of reciprocal clock and cion influence has been brought to the front by the recent use of vegetatively propagated rootstocks, or clons, in place of seedling rootstocks upon which to bud and graft fruit trees. Theoretically, vegetatively propagated rootstocks, since each type has been propagated from an original plant by such vegetative means as cuttings and layers, should be more uniform than seedling rootstocks, no two of which are of identical genetic make-up.
These vegetative types have been found to produce different effects upon the cion worked upon them, such as dwarfing, age of bearing, season of fruit ripening, and productivity (49, 52, 61, 68). The emphasis thruout has been placed upon the effect of the rootstock upon the cion, altho Hatton, Grubb, and Amos (23) in 1923 noted that the cion might have an important influence upon the stock, while Barker (4, 5) in 1927 and in 1930 also pointed out the importance of the cion in relation to the stock. Grubb (14) recorded that the intermediate stem-piece might also have an effect in double-worked trees.
In 1927, however, Swarbrick and Roberts (62) focused major attention upon this problem by showing that the cion, and particularly the stem portion of the fruit tree, might in some cases dominate root development to such an extent as to negative the effect of the rootstock upon the cion. They seemed to find, moreover, that budded trees, that is trees in which the cion variety is inserted upon the stem of the rootstock, did not show the same dominance of cion over seedling root as was exhibited by root-grafted trees in which the cion variety is inserted upon the root of the rootstock. They pointed out that in the case of the budded trees there would be a portion of rootstock stem lying between the root portion of the rootstock and the cion variety, whereas in the case of the root-grafted trees there would be no stem excepting that of the cion variety. It was reasoned, therefore, that, ''Obviously stock influence is due to the stem portion of the stock".
Reasoning one step further, they concluded that the uniformity of trees propagated upon vegetatively propagated rootstocks was due to the uniform stems which such stocks would have in contrast to the variable stems of seedling rootstocks. Thus, Swarbrick (58) says, "It is legitimate to argue that a large proportion of the variation that supposedly arises from the use of seedling rootstocks is a direct result of the English custom of working some 6 to 8 inches high upon the seedling stem. ... The trees, because of this interposed stem-piece, have the same make-up as double-worked trees."
In order to test this so-called "stem effect" further, and in order to determine its nature and particular mode of action, Roberts (46. 47, 48) double-worked many varieties of apples, introducing a piece of stem several inches in length from a known variety, or clon, between the top cion and the rootstock. From this series of experiments, he classified several varieties and clons of apples into four groups as judged by their effect upon the rootstock when used as intermediate stem-pieces, namely, dominant, neutral, intermediate, and incompatible. "Dominant" stem pieces prevented the top cion from modifying the rootstock and produced roots typical of the intermediate variety. "Neutral" stem pieces permitted the top cion to modify the rootstock. "Intermediate" or "medium" stem pieces could not be classified definitely as either dominant or neutral. In their work, Roberts and Swarbrick emphasized that the effect upon the rootstock is in many cases qualitative, that is, not only is the total amount of root altered, but the shape and form as well.
Quantitative vs. qualitative effect.—The work of Hatton (19) and his co-workers, on the other hand, employing clonal rootstocks of recognized effect upon the cion, indicated that the reciprocal effect of cion on stock is quantitative and not of great magnitude. Thus, Vyvyan (73) says,
"The root system of clone stocks, such as No. II and No. VI, when stem-worked with cion varieties, such as Beauty of Bath and Grenadier, retains each its own distinct morphological characters approximately constant, irrespective of the variety used as cion, although the cion variety may have a marked influence on the quantity of roots and hence have some indirect effect on the relative amounts of large and small roots."
Amos, et al. (1) call attention to the fact that a large root system has a lower percentage of fibrous roots than a small one, so that the use of different cions may indirectly affect the percentage of fibre on the roots by altering the size of the whole system. They show that in the maiden year, the total weight of roots is characteristic of the stock, and they say that the differences produced by the cion are small in comparison with the differences between the root systems when unworked, showing that the rootstock thus maintains its varietal character irrespective of any influence of the cion variety. Rogers (51), concurs in these results and concludes from his work with pears that again the cion influence is quantitative and not qualitative.
Finally, from experiments designed expressly to show the effect of an intermediate stem-piece, Vyvyan (73) demonstrated that the dominance of the rootstock still existed even when the cion variety was root-worked onto piece-roots of the stocks instead of stem-worked onto rooted stems. Likewise, Hatton (22), reporting further upon this problem of employing clonal rootstocks, showed a rootstock influence even when clonal piece-roots were used, thus indicating that the effect upon the material with which he was working was not alone a "stem" influence. Furthermore, he showed that budding and grafting produced the same results. Neverthless, he recognized that part of the influence was in the stem of the rootstock, "The path by which the materials, collected by the roots, pass to the cion."
PURPOSE OF THIS PUBLICATION
It is the purpose of this publication to discuss the question of stock and cion relation for New York conditions; to determine whether the effects are measurable, whether they are permanent, and whether they are commercially important to the fruit and nursery interests of the State; and to present new data that may help to clarify the situation, with particular reference to the effect of the cion upon the root.
MATERIALS AND METHODS
In January, 1928, 1,800 French Crab piece-roots were root-grafted with 14 varieties and clons of apple, in 43 combinations of single- and double-working. These seedling rootstocks from which the piece-roots were selected for size and uniformity were imported from France to the American nursery trade and were typical of the rootstocks upon which American apple trees are propagated.
The tongue graft was used and the unions wrapped with adhesive grafting tape, as shown in Fig. 2. Cion wood and root-piece were carefully matched for size, so that uniting parts were matched on all sides. While the arrangement of the top bud of the cion directly above the matched side of the graft may be a factor where either the cion or the stock is appreciably larger in diameter than the other (32, 45, 59), it is not an important factor when the stock and cion are nearly identical in diameter. Nevertheless, the top was placed immediately over one carefully matched side so as to eliminate this factor.
|FIG. 2.—METHOD USED IN MAKING SINGLE-WORKED AND DOUBLE-WORKED GRAFTS|
Forty single-worked grafts each were made of 12 apple varieties and 2 clons, namely, Hyslop, Jonathan, Liveland, Northern Spy, Northwestern Greening, Oldenburg, Tioga, Titovka, Wagener, Wealthy, Winesap, and Whitney and Malling IX and Malling XIII. These varieties were selected because noticeable differences in root development have been observed in the American nursery trade when they have been used as cions. Of the two clons chosen, the one is a dwarfing stock and the other a free-growing or standard stock.
In addition, 40 double-worked grafts of each were made of 29 combinations between these varieties and clons in which a 2-inch stem-piece of a given variety and clon was inserted between the French Crab root-piece and the top cion, as shown in Fig. 2. The combinations were arranged so as to have the top cion of one variety used as an intermediate stem-piece in one combination, and the reciprocal arrangement in another as follows:
|Jonathan on Malling IX
Malling IX on Jonathan
|Northern Spy on
Liveland on Northern Spy
|Malling IX on Tioga
Tioga on Malling IX
|Wealthy on Hyslop
Hyslop on Wealthy
|Jonathan on Malling
Malling XIII on Jonathan
|Wealthy on Wagener
Wagener on Wealthy
|Malling XIII on Tioga
Tioga on Malling XIII
|Wagener on Hyslop
Hyslop on Wagener
|Whitney on Northwestern Greening
Northwestern Greening on Whitney
|Northern Spy on
Oldenburg on Northern Spy
|Whitney on Titovka||Northern Spy on
Winesap on Northern Spy
|Northwestern Greening on
Northern Spy on Northwestern Greening
|Winesap on Titovka
Titovka on Winesap
|Oldenburg on Liveland
Liveland on Oldenburg
These combinations were selected so as to place a variety which produces coarse roots, as Oldenburg, with one which produces fine roots, as Liveland or Northern Spy; a dwarf clon, as Malling IX, with a standard clon, as Malling XIII; a variety which produces spreading roots, as Wealthy, with a variety that produces descending roots, as Hyslop; a vigorous-growing variety, as Tioga, with a less vigorous-growing variety, as Jonathan; and so on.
By this method of double-working, in which the unions are made at one time during the dormant season, no foliage is permitted to develop upon the intermediate stem-piece. The importance of this point will appear later.
The grafts were lined out on Ontario clay loam soil, typical nursery land of western New York, in the spring of 1929 and grown under careful nursery management, including proper spraying. The plants were set with the cions above the soil level so that no roots might form from either the top cion or the intermediate stem-piece, and care was taken thruout the life of the experiment to prevent any roots forming.
Characteristic of grafts under western New York nursery conditions, the first year's growth was small, the 1930 growth made salable 1-year trees, and the 1931 growth made salable 2-year trees. At the end of the first season the trees were dug and observations made of their root development. They were then replanted. In the fall of 1931 the trees, now 3-year trees from the graft, were again dug and observations made of the top growth, root development, and type of union.
The work has been conducted at Geneva in western New York in one of the large nursery producing sections of America, and the grafting and general care of the plants has been done by practical nurserymen, thus providing uniform conditions and eliminating personal factors.
Fig. 3 shows the unions of a double-worked tree, showing both externally and in longitudinal section the almost perfect unions between rootstock and intermediate stem-piece and between intermediate stem-piece and top cion.
It has been found very difficult to describe and catalog the results. The characters analyzed do not lend themselves well to measurement. Such terms as abundant, scraggly, stringy, coarse, fine, fibrous, spreading, outward spreading, downward spreading, and the opposites, have been used to describe the root system, yet they are not exact. Angle of branching has been measured but is far from satisfactory. Differences have been noted in size and length of roots, proportion of fine roots, coarse roots, and color, but with questionable value.
|FIG. 3.—LONGITUDINAL SECTION AND EXTERNAL VIEW OF A SUCCESSFUL UNION OF A DOUBLE-WORKED TREE, INVOLVING THREE SEPARATE INDIVIDUALS, NAMELY, A ROOTSTOCK, AN INTERMEDIATE STEM-PIECE, AND A TOP CION.|
Photographs are probably most successful, yet here too a close examination of the living material in which a certain set of characters combine reveals a definite effect which the photograph does not show. The difficulty is something like that in describing a variety of fruit. One may recognize it and separate it from other varieties, but he finds difficulty in recording the details by which he identifies it. The reader will find that a close study of the photographs will gradually reveal details which he did not at first observe. Even then, as Hatton (22) says, a rootstock influence cannot be detected solely by measurement. It expresses itself usually in a number of minor effects, which taken together give a characteristic expression for that rootstock.
Furthermore, just as with a single specimen of a variety of fruit, so the characteristics of a certain stock or cion effect may or may not be well shown by a single tree, yet taken as a group they show general characteristics which separate them readily from other groups. For this reason, the trees from any one treatment have been placed in a bundle and photographed together.
FIRST YEAR RESULTS
The results of the first season's growth of both single-worked and double-worked grafts will not be discussed in detail nor will photographs of all combinations be shown because of necessary economies.
Effect of top cion and intermediate stem-piece upon root development.-Figs. 4, 5, and 6 are representative photographs from the 43 combinations in the experiment. They show a general disposition for the top cion to dominate the root system of the rootstock, whether an intermediate stem-piece of some other variety has been introduced or not.
For example, in Fig. 4, using Hyslop as the top cion, the root systems are all of a "scraggly" type, with a scarcity of lateral root development, regardless of the fact that in one case a stem-piece of Wealthy and in another case a stem-piece of Wagener has been introduced between the Hyslop top cion and the rootstock.
In Fig. 5, involving Liveland as the top cion, root development has been much greater than with Hyslop, and the proportion of fine lateral roots is so much greater as to be called "hairy". The interposition of Northern Spy and Oldenburg stem-pieces between the Liveland top cion and the rootstock has not altered this situation.
|FIG. 4.—HYSLOP—WEALTHY—WAGENER GRAFTS.|
Note "scraggly" root system with scarcity of lateral development whether single-worked (left) or double-worked with intermediate stem-piece of Wealthy (center) or of Wagener (right). The disposition is for the least vigorous grafts (center) to develop roots descending most sharply, the most vigorous graft (left) to develop most spreading roots, and the graft intermediate in vigor (right) to develop roots intermediate in angle of descent. Compare with Figs. 6 and 7.
|FIG. 5.—LIVELAND-NORTHERN SPY—OLDENBURG GRAFTS.|
Note the high proportion of fine roots, giving a "hairy" appearance to root system of all grafts with Liveland as the top cion, whether single-worked (left) or double-worked with an intermediate stem-piece of Northern Spy (center) or of Oldenburg (right). Compare with Figs. 4 and 6.
|FIG. 6.—OLDENBURG—LIVELAND GRAFTS.|
With Oldenburg used as the top cion (Fig. 6), the development of the root system has been intermediate but has seemed similar whether a stem-piece of Liveland was inserted between top cion and root or not.
Vigor of top and size of root.—A closer examination of these three photographs will show a correlation between the vigor of the first season's growth and the size and total number of roots produced. The top cions of the grafts involving Hyslop (Fig. 4), for example, have made least increase in diameter, and the root development is poorest; the top cions of the grafts involving Liveland (Fig. 5) have made the largest increase in diameter, and the root development is best; while with Oldenburg (Fig. 6), the growth of the top is intermediate and root development likewise. In this respect, then. the first season's growth seems to show a quantitative rather than a qualitative difference in the effect of the cion upon the root.
Vigor of top and angle of root development-These photographs illustrate another major point, namely, the disposition for the least vigorous grafts in any group to have roots which develop at an acute angle with the vertical, whereas the most vigorous grafts seem to be correlated with a more spreading habit.
In Fig. 4 the central graft of the three has made weak growth and has sharply descending roots. The graft to the right has math greater growth and has roots which are less sharply descending, while the graft to the left has made most growth and has roots with still more spreading habit. This same situation is discernible in Figs. S and 6.
The authors have made this same general observation from their work in the production of seedling rootstocks (65). There is sonic evidence to believe, however, that time of bud start may be the determining factor rather than "vigor" alone.
Dwarf stocks.--Altho the major evidence from the first seasons growth is to the effect that the intermediate stem-piece is not so important as the top cion in determining the growth of the plant and the development of the roots, yet in the case of Malling IX (Jaune de Metz), used either as a top cion or as an intermediate stem-piece, the result is to dwarf the top, dwarf the roots, and to produce the sharply descending roots already noted for more weak-growing plants. This factor will be discussed further in connection with 3-year-old trees, but it is important to observe that the dwarfing effect of dwarf top cion and intermediate stem-piece is observable during the first season's growth of the graft.
RESULTS WITH 3-YEAR-OLD TREES
The trees when dug in 1931 were the size and character of 2-year-old nursery trees. The first year, as explained, grafts under the conditions of this experiment make but little growth so that the second season they develop into a whip from a single bud and the third season they develop lateral branches on the 1-year-old central axis exactly as does a 2-year-old budded tree. Table 1 shows the number of individuals surviving 1 year, the number surviving 3 years, the mean height of the top cion, and the mean height of the salable trees.
The proportion of individuals which survived the experiment give some indication of the congeniality of varieties. Mean height of the top cion is not as good a criterion as is desired of the performance of that cion on a particular intermediate stem-piece and rootstock for the reason that in some combinations there were often several small trees which survived and which served to lower the value for the group, whereas in other combinations only the vigorous individuals survived and served to give an unusually high rating for height of top cion. The column designated "Mean Height of Salable Trees" is intended to show the height of the most successful trees in any given combination, and altho involving the factor of personal judgment, these figures are considered by the authors to be the most significant for comparison between different combinations.
While the growth of the top cion and the type of unions have not been overlooked, yet the principal attention has been given to the effect of the top cion and of the intermediate stem-piece upon the growth and configuration of the rootstock.
Figs. 7 to 16 show the shape and character of the top cion, the unions, and the shape and character of the roots.
Congeniality of reciprocal graft unions.-It must be noted that while in most instances the results of reciprocal unions of cions have resulted in similar unions and growths, as Northwestern Greening on Northern Spy and Northern Spy on Northwestern Greening (Fig. 12), yet in other instances there have been a good union and good growth in one direction and a poor union and poor growth in the reciprocal graft, as Northern Spy on Winesap and Winesap on Northern Spy (Fig. 14). A glance at Table 1 will show this same
TABLE 1.—PERFORMANCE OF APPLE VARIETIES, SINGLE-WORKED AND DOUBLE-WORKED ON FRENCH CRAB ROOTS.
|NUMBER OF PLANTS
SURVIVING 1 YEAR
|NUMBER OF PLANTS
SURVIVING 3 YEARS
OF TOP CION, CM.
OF SALABLE TREES, CM.
|Northwestern Greening||Northern Spy||40||30||26||121.92||144.78|
|Northern Spy||Northwestern Greening.||40||29||26||102.67||125.73|
|FIG. 7.—HYSLOP—WAGENER RECIPROCAL GRAFTS.|
Note the coarse downward roots of single-worked Hyslop (left), the less coarse and more spreading roots of single-worked Wagener (right), and the uncongeniality and dwarfing of double-worked Hyslop and Wagener (center).
situation with regard to the number of plants surviving 3 years. In most combinations the number of individuals surviving are similar. In the Northwestern Greening and Northern Spy combinations, for example, 26 individuals survived in each of the two reciprocal unions. On the other hand, in the case of the Winesap and Northern Spy combinations, 15 of the grafts survived when Northern Spy was the top cion and only 8 when Winesap was the top cion.
|FIG. 8.—HYSLOP—WEALTHY RECIPROCAL GRAFTS.|
Note the coarse, downward roots of the single-worked Hyslop (left) the finer, more spreading roots of Wealthy (right) ; the root system characteristic of Hyslop top cion, altho double-worked with Wealthy intermediate stem-piece (left center); and the root system characteristic of Wealthy top cion, altho double-worked with Hyslop intermediate stem-piece (right center).
A factor may be the relation between the rootstock and the cion. For example, it is an accepted horticultural practice (14) to double-work pear varieties that do not readily unite with the quince, using a cion variety which unites well with the quince and top-working this cion, in turn, with the desired variety.
|FIG. 9.—MALLING IX (DWARF) AND MALLING XIII (FREE-GROWING) TOP CIONS DOUBLE-WORKED WITH INTERMEDIATE STEM-PIECES OF JONATHAN AND TIOGA.|
Note the uniform vigor, height, and growth habit of trees with Mailing XIII top cion, whether single-worked or double-worked with an intermediate stem-piece of either Jonathan or Tioga (left). Also note the uniform dwarfing and growth habit of trees with Mailing IX top cion, whether single-worked or double-worked with an intermediate stem-piece of either Jonathan or Tioga (right).
|FIG. 10.—TIOGA AND JONATHAN TOP CIONS DOUBLE-WORKED WITH INTERMEDIATE STEM-PIECES OF MALLING IX (DWARF) AND MALLING XIII (FREE-GROWING).|
Note the dwarfing of both Jonathan and Tioga (right) when double-worked with intermediate stem-piece of Malling IX, and the comparative height and vigor of both Jonathan and Tioga when double-worked with intermediate stem-piece of Malling XlII.
|FIG. 11.—LIVELAND—OLDENBURG—NORTHERN SPY RECIPROCAL GRAFTS.|
Note the coarse roots and sparsely branched top of Oldenburg trees (right) regardless of double-working with intermediate stem-pieces of Northern Spy and Liveland, also the finer roots and more branched top of Liveland trees (left) regardless of double-working with intermediate stem-pieces of Northern Spy and Oldenburg.
those involving Oldenburg. With Oldenburg as the top cion, the roots were reddish in color even tho an intermediate stem-piece of another variety was interposed between the top cion and the rootstock. On the other hand, combinations which included Oldenburg as the intermediate stem-piece with some other variety as the top cion, did not show any reddish coloring.
|FIG. 12.—NORTHWESTERN GREENING—NORTHERN SPY RECIPROCAL GRAFTS.|
|FIG. 13.—NORTHWESTERN GREENING—WHITNEY RECIPROCAL GRAFTS.|
Note the tall, fine, well-branched tops of Northwestern Greening trees and the sparse, short, spreading roots regardless of double-working with intermediate stem-piece of Whitney (right). The shorter, stockier, less-branched tops of Whitney and the more abundant, longer, downward, finer roots, regardless of double-working with intermediate stem-piece of Northwestern Greening (left) are also to be noted. Congeniality, smooth unions, and vigorous top growth are apparent.
|FIG. 14.—NORTHERN SPY—WINESAP RECIPROCAL GRAFTS.|
Note the downward direction of roots of Northern Spy trees (left) regardless of double-working with intermediate stem-piece of Winesap and the more outward direction of roots of Winesap trees (right) regardless of double-working with intermediate stem-piece of Northern Spy. The swollen unions and uncongeniality of Winesap double-worked on Northern Spy may also be observed.
|FIG. 15.—TITOVKA—WINESAP GRAFTS.|
Note the poor growth of Winesap when double-worked with intermediate stem-piece of Titovka, as with Northern Spy in Fig. 14.
The observation that roots of Oldenburg trees may be red is not new. The twigs, central axis, and leaves develop much red coloring, and nurserymen have repeatedly observed the reddish cast in the roots as well.
Whether this is a transfer to the root system of anthocyanin developed in the top cion or whether the root system develops anthocyanin because of general physiological conditions, such as growth and maturity factors, controlled largely by the top cion, is a question for debate. Hofmann (27) has shown that with a red strain of Refugee bean grafted on a green strain no pigment is found in the roots. Nevertheless, the fact remains that when Oldenburg is used as a top cion, the roots take on a reddish appearance, even when an intermediate stem-piece of another variety is interposed between the Oldenburg top and the rootstock.
|FIG. 16.—WAGENER—WEALTHY RECIPROCAL GRAFTS.|
Note the swollen unions and uncongeniality of Wagener and Wealthy when double-worked (center) as compared with single-worked Wagener (left) and Wealthy (right).
From these data it can he seen that under the conditions of this experiment the top cion has had in some instances an appreciable influence upon the growth and character of the rootstock and of the grafted plant, and also that the intermediate stem-piece in other instances has likewise had an effect upon the entire plant. Furthermore, two major factors have seemed to dominate in this situation. namely, the nature of the top cion and the compatibility of the union.
INFLUENCE OF THE TOP CION
In combinations in which the graft unions appear smooth and congenial so that the union in itself is not a limiting factor, it has been the top cion which has dominated the shape and character of the rootstock, and not the intermediate stem-piece.
Effect of cion foliage upon development of the stock-In seeking for an explanation as to how the cion brings about an effect upon the rootstock, attention is at once directed to the differences that exist in foliage development between various cion varieties and the possible part that the foliage may play. From time to time reports have been made by practical horticulturists that when a fruit tree has been top-worked with another variety, the fruit borne by the cion has been affected by the stock, and vice versa. Trowbridge (64) reported in 1881 that when a red striped variety of apple had been grafted onto a tree bearing green apples that the apples borne upon the stock below the cion were like the fruits of the cion. Pease (40) in 1906, suggested by a rhetorical question that a fruit borne on a shoot appearing below a cion might he affected by the cion.
Altho little scientific credence was given to these sporadic reports ... it is interesting to see how closely the theoretical explanation then given approaches the explanation by present-day experiment. Talbot (63) in 1879, said, 'The sap elaborated in the leaves of the stock passes so strongly into the grafts as to change the characteristics of the wood, and here is where the character of the fruit is determined." Booth (6), in 1914, in listing seven points in grafting as judged from the standpoint of plant anatomy and physiology, included the possible selective effect of the leaves of the cion upon the stock. Hedrick (24), in 1915, stated specifically, "It may he that the food elaborated by the foliage of the cion is different from that which the stock would have had with its own foliage."
Heinicke (26), in 1927, supplied more exact evidence in support of these statements. He found that when a McIntosh apple tree had been top-worked with a Tompkins King cion that the McIntosh fruit produced on the stock below the Tompkins King cion was in some cases so much like fruit of Tompkins King as to be easily mistaken for it. He used other varieties, some of which showed the effect of a cion upon the fruit of the stock and some of which did not, suggesting a varietal difference in behavior in this regard. These findings, coupled with the data of Haller (17) showing that the leaves on one portion of a tree may support the development of a fruit several feet distant on branches without foliage, indicate that the foliage of the cion may, in some instances, have an important bearing upon the performance and development of the stock upon which it is worked.
In this connection it may be well to point out some particular differences in foliage development which might conceivably play a large part in cion effect, such as time of bud start, size of foliage, and chlorophyll development. Buds of the Kieffer pear, for example, start earlier in the nursery in the spring than buds of Bartlett and Seckel on the same rootstocks. Kieffer buds will have grown to a height of 3 inches when Bartlett and Seckel buds are just starting; and Baldwin buds will be leafed out when Northern Spy buds are still unbroken. Wolf River foliage is much larger than Jonathan, and Oldenburg foliage is much greener. These factors, and many others, involving photosynthesis and nutritional balances might play an important part in the effect of the cion on the stock.
Importance of method of double-working.—Some of the differences in results that have been obtained by workers in this field may possibly be due to the foliage effect as brought about by the method used in double-working. When double-working is accomplished by whip grafting with one cion one season and budding or grafting this cion with the other variety during the subsequent growing or dormant season, foliage is permitted to develop on the intermediate stem-piece during part, if not all, of the first growing season. Knight (31), in evaluating, his own work, has carefully pointed out this difficulty, showing that the intermediate stem-piece thus develops foliage and grows for 1 year before the top cion takes the ascendency so that the former may have influenced the rootstock before the top cion became operative.
On the other hand, if double-working is accomplished during the dormant season, as was done in the experiment reported here, no foliage is permitted to develop upon the intermediate stem-piece. It is not clear in all cases that have been reported whether double working has been done during the dormant season or whether the cion variety has been budded upon the intermediate stem-piece during the following growing season. In some instances, at least, the latter method has been used (46). In that event, the question is immediately raised as to what effect the foliage of the intermediate stem-piece would have upon the root development of the rootstock prior to the time that the bud from the cion variety had developed foliage and had become the dominant top of the new plant. In view of the part which cion foliage may play in determining the performance of the stock (26), this point would seem to be of importance.
INFLUENCE OF THE INTERMEDIATE STEM-PIECE
While the intermediate stem-piece has seemed not in itself to have affected the root directly under the conditions of this experiment, yet it has in some instances affected it indirectly thru a decided effect upon the entire plant, as by dwarfing it.
Effect of dwarfing stocks.—Where Malling IX, a dwarf stock, was used as the intermediate stem-piece, the result was to dwarf the entire plant even tho the top cion was of a vigorous variety (Fig. 10). In fact, the use of Malling IX in any position results in a dwarf plant. Used as the top cion it has produced dwarf plants (Fig. 9); used as the intermediate stem-piece it has produced dwarf plants (Fig. 10); and, of course, used as a rootstock it results in dwarf plants. Roberts, too, noted that dwarfing stocks behaved differently than other stocks used by him, observing that (46),
"In striking contrast to the marked influence which the cion variety has upon root formation in the case of seedling piece-root grafts, there seems to be practically no such influence in the case of vegetatively propagated dwarfing stocks, * * * dwarf varieties remain dwarfish on vigorous stocks."
Structural nature and physiological relations of graft unions.-In seeking an explanation as to how the intermediate stem-piece may affect the plant, particularly with reference to dwarfing stocks, attention is immediately focused upon the graft union because of the fact that the unions with dwarfing material in this experiment were gnarled and rough by contrast to the smooth unions of vigorous trees.
What has been learned anatomically and structurally concerning the nature of the union between stock and cion makes a valuable chapter in the analysis of the entire problem of stock and cion relation. Hoopes (28), in 1873, emphasized what is perhaps the most significant point concerning unions, namely, that individual cells of stock and cion remain distinct-never uniting in the sense that two cells become one. He makes the nice distinction that while the individual cells of the stock end of the cion may remain distinct, yet a collection of cells taken at the point of union may include both stock and cion.
Other workers have subsequently emphasized this fact (7, 41, 42, 74), considering the so-called "union" as a dove-tailing of elements of both stock and cion. In other words, there is no union in the sense that gametes unite to form the zygote. The cells of the stock and of the cion remain distinct, even tho the xylem elements may he arranged in continuous rows and even tho the xylem rays may extend across the line of union. The so-called "union" of stock and cion is merely an anastomosis of various degrees.
It is also important to recognize that there are varying degrees 1 continuity of tissue. Waugh (74) has pointed out that the weakness of unions results nearly always from physiological incompatibility of the don and the stock. Proebsting (41, 42) has made a critical study of tissue at the point of contact between stock and non. He pictures a strong union as one in which the vascular elements of both stock and cion are dove-tailed strongly together, as illustrated by the French prune on Muir peach. Furthermore, he recognizes four types of structural defects in stock and cion relations, as follows
Bradford and Sitton (7), likewise, in speaking of the cause of dwarfing of apples and pears, conclude that the cases of dwarfing are associated with phloem discontinuity, the effect appearing to be reciprocal. The stock is dwarfed and in turn dwarfs the top.
Ravaz (43), too, emphasizes the physical nature of the union in considering the reciprocal effect of stock and cion among grapes, altho he does not recognize the use of the word "reciprocal" in the sense that it is employed here. He says, "All the variations of colour, shape, size, flavor of the berries, etc., observed in grafted vines occurs equally in the same varieties grown on their own roots. Phylloxera resistance is not altered nor is there any modification in the color or flavor of the fruit of those varieties of Vitis vinifera, which are grown on American varieties. The peculiarities which show in the graft and the internal structure at the graft junction are the result of the different speeds of growth of stock and cion. This inequality plays an important part in the fertility and ripeness and quality of the product. Similar effects are produced by annular incision or ligature. The growth of the grafted variety is dependent on that of the stock, and vice versa."
Chandler (9) and Gardner, Bradford, and Hooker (11) have brought together the observations and records of earlier workers in this field, and in their interpretation they, too, emphasize the physiological aspect of the problem. Thus Chandler (9) says:
"When the cion is of the more vigorous growing form, the carbohydrate supply for the roots should be greater. In other words, the growth of the stock should be influenced by the cion. Since ... certain roots supply and are supplied by certain branches, it might he expected that the branching habit of the top would influence that of the roots, at least to some extent. *** Water, mineral nutrients, and carbohydrates may pass from the tissue of one to that of the other. No doubt the tissue of one may dwarf or stimulate the other by its weak or vigorous growth. But there seems to be no convincing evidence of a relation other than nutritional or physical."
Gardner, Bradford, and Hooker (11) conclude,
"Just as in the case of stock on cion, in considering the influence of cion on stock it is not necessary, so far as fruit plants are concerned, to predicate any direct effect other than on vigor. Every other influence that has been established or attributed can be explained as exercised indirectly through vigor and can be placed on a quantitative basis. This action on vigor may he direct when the two parts to the graft are congenial and make a good union, or it may be indirect when there is apparent uncongeniality and the union is poor. Qualitative influences, such as the passage of alkaloids across the graft, or the barring of inulin by the graft, are not necessary to explain any observed phenomena resulting from grafting in fruit plants."
"Stem effect vs. union effect."—There is much evidence, therefore, which points to the nature of the ''union," or rather "the degree of union," as a very important consideration in stock and cion relation. In relations between stocks and cions in double-working (14), the effect of an intermediate stem-piece has been thought of as a "stem-effect," as tho the stem itself were responsible for the altered behavior of top cion or root stock. The evidence from experiments reported in this publication, together with the critical observations just recorded in regard to the structure of the graft union, gives credence to the view which the authors wish to emphasize, namely, that it is the region of contact between combining tissues, the "union'' rather than the ''stem" which is the more important consideration.
BUDDING VS. GRAFTING
The data reported in this publication deal only with grafted trees, and while they show clearly the reciprocal stock and cion influence for grafted plants, they do not shed any light on the situation for budded trees. Nevertheless, there is abundant evidence that similar stock and cion influences exist in budded nursery stock, as already enumerated. Moreover, Gardner, Bradford, and Hooker (11) expressly picture the cion effect in budded nursery stock from a photograph supplied by Shaw, and Maney (27) records similar data. One need only visit the nursery sections of America, such as the nursery center of western New York in which stem budding is the method of propagation, to see nurserymen selecting varieties of trees in bins solely by the exposed root development of seedling rootstocks stem-budded to the desired variety. The cion effect, therefore, has not been eliminated by stem-budding, nor by the portion of seedling rootstock stem lying between the cion top and the root portion of the rootstock. In this respect, observations of stem-budded trees agree with the findings with grafted trees in this experiment, namely, that when the union is compatible, it is the top cion and not the intermediate stem-piece which affects the root system.
High vs. low budding.—Another question is whether height of budding may alter the cion influence. General observation in nursery sections where it is the custom to bud slightly higher than in others still show the effect of cion upon the rootstock. Moreover, in 1927, the senior author budded the Cortland apple onto 250 French Crab rootstocks in three positions, namely, in the root of the root stock, in the stem of the rootstock 2 inches above the ground line and in the stem 6 inches above the ground line. When the trees were dug as 2-year trees, the cion effect could be seen in all three lots. Later, the authors repeated this test, using apples, pears, cherries, and roses. The general observation was that height of budding did not influence cion effect except as it might affect winter exposure of the buds, time of bud start, and coverage of the bud by soil. For example, very low budding resulted in slower bud start of apples for the reason that the buds become covered with soil and soil temperature is lower than air temperature in early spring, thus delaying the start. On the other hand, low budding of roses has been superior to high budding for the reason that the buds are protected from winter cold by soil coverage and therefore respond with greater vigor the following spring.
Furthermore, when a plant is high-budded the amount of top removed when the stock is cut back to just above the bud is quite different than when low-budded. The problem thus becomes one of degree of pruning and physiological balance rather than one of cion influence as recognized in this publication.
DIFFERENCES IN PLANT MATERIALS, CLIMATE, AND LOCAL FACTORS
The apparent differences in results from different sources, principally England and America, may be explained in part by differences in plant materials and in climatic factors (8). For example, rootstocks employed by American investigators have been preponderantly seedling in origin and selected by the exigencies of commercial competition for their vigor and congeniality with American varieties of fruit trees. Quite by contrast, many of the rootstocks used by English workers have been vegetative selections, having been perpetuated for some particular effect which they have shown to exert upon the cion, such as various degrees of dwarfing. With such unlike material, it is not surprising, that there should be differences in results. The danger, of course, does not lie in differences in results, but in attempting to generalize.
As regards climate and local conditions, Irwin (29) shows that even within relatively short distances, as between East Malling and Long Ashton in England, the growth curves for certain plants may differ. At East Malling, apples make a good early growth (May) but make a poorer late growth (July), whereas at Houghall the early growth is poorer but the late growth is greater. The Bramley apple makes a greater total growth than the Worcester apple, but the advantage which Bramley has over Worcester is greater at Long Ashton and Osgoodby than at East Malling and at Houghall because the seasonal factors at the former localities favor the growth curve for Bramley. Seasons, too, many differ sufficiently to upset the "normal" condition, as in 1926 when conditions were more favorable to Bramley and less favorable to Worcester, while in 1927 and 1928 the reverse was true.
Barker (5), too, singles out this factor. The East Malling rootstocks, for example, do not exhibit such striking differences at Long Ashton as at East Malling, "Doubtless due to the drier conditions at the latter."
It is a common observation in American nurseries (67) that certain varieties of fruit trees may make a greater height in some seasons than in others, and vice versa. It is not surprising, therefore, that there might be differences in results in England and in America, quite aside from differences in technic and in personal factors.
From the data presented here it is shown that the cion may definitely affect the development of the rootstock even tho an intermediate stem-piece of some other variety may be interposed between the top cion and the rootstock. It is also shown that in some instances the intermediate stem-piece may have an appreciable effect upon the entire plant, as in the use of a stem-piece of dwarfing stock. Also, it is known generally that a rootstock may by itself have an important effect upon the entire plant.
The rootstock may play a part thru selective absorption. The top cion may play a part thru elaboration of plant food and thru rate of transpiration. The union may play a part thru interruption of transfer of nutrients, as by the introduction of parenchymatous tissue and even "bark" between the consorting parts. Added to these is the relative metabolic activity of different parts governed by genetic make-up. Besides all these there are such local factors as temperature, rainfall, and the optimum condition for any one of the consorting parts. Many aspects of the problem, therefore, seem largely physiological.
The question, then, is no longer which one part dominates the entire plant, since all have been shown to have marked influence at one time or another. The problem is rather one of relation and degree, as Barker has emphasized (4). The rootstock may be the limiting factor one time, the union another, and the top cion another. For example, if a dwarfing stock is used, the dwarfing character seems to dominate, whether the dwarfing stock is used as rootstock, intermediate stem-piece, or top cion. On the other hand, a top cion may be the limiting factor in other combinations as the result of tender roots induced by the late-growing tendency of the top cion variety.
Moreover, the rootstock may affect the top cion in one way, the cion may affect the root in the same direction or in another, and the union may affect both rootstock and top cion in the same or still other directions. The combination and intergrading of these factors give what Hatton has so aptly termed the "build-up" of a fruit plant, and no one of these factors should be entirely disregarded.
From the practical viewpoint, with reference to the present-day problems of the American orchardist, it can be said that, altho the cion influence may occasionally be a limiting factor, it has so far not been a major problem. In general, the stock influence has been the greater of the two.
Fruit trees are composed of two individuals growing together as one, namely, a rootstock and a cion budded or grafted thereon. Horticulturists have been forced to resort to this method of propagation because fruit trees will not come true from seed, they are difficult to propagate by vegetative means, and a variety may perform better upon other roots than its own.
The American methods of propagation are whip-grafting upon piece-roots and budding upon seedling rootstocks, the latter method being much more generally used, even to the exclusion of whip-grafting in some sections.
Various problems are raised by these methods of propagation, such as effect of the rootstock upon the plant, congeniality of stock and cion, and, reciprocal effect of the cion upon the rootstock involving such factors as hardiness, disease and insect resistance, soil adaptation, and longevity.
That the stock may have an effect upon the behavior of the cion is well known. On the other hand, the effect which the cion may have in its turn upon the stock has been less understood, altho the fact is not new. American horticultural literature records observations as early as 1846 that the cion may affect the development of the rootstock, and other instances have been recorded since. It has been shown that the cion may affect the root in other ways than in quantity, direction, and form of roots, such as in longevity, hardiness, chemical composition, and color, flavor, and shape of fruit.
An entirely new and important aspect to the problem of reciprocal stock and cion influence has been brought to the front by the recent use of vegetatively propagated rootstocks, or clons, as improvements over seedling rootstocks, upon which to bud and graft fruit trees. These vegetative types have been of value because of different effects which they have had upon the cion worked upon them, such as time of fruiting, dwarfing, and so on. The emphasis has been placed upon the effect produced by the rootstock.
Still more recently the indication has been that the cion, and particularly the stem portion of the fruit tree, might in some cases dominate root development to such an extent as to negative the effect of the rootstock upon the cion. Some workers seemed to find that budded trees, in which the cion variety is inserted upon the stem of the rootstock, seemed not to show the same dominance of cion over seedling root as was shown by root-grafted trees in which the cion variety is inserted upon the root of the rootstock. Not only did the cion seem to alter the quantitative development of roots, but also the qualitative development. An intermediate stem-piece inserted between stock and cion by double-working seemed to be the controlling factor. On the other hand, other workers seemed to find that rootstock influence was seldom masked by cion influence and that cion influence was quantitative in nature and not qualitative.
The results reported in this publication deal with 1,800 French Crab piece-roots root-grafted to 14 varieties and clons of apple in 43 combinations of single- and double-working. The tongue graft was used, both single- and double-grafts being made during the dormant season, thus permitting no foliage to develop upon the intermediate stem-piece.
It has been found very difficult to describe and catalog the results because the characters analyzed do not lend themselves well to measurement. Photographs have been used largely, therefore, to record the results.
Photographs from the first season's growth of both single- and double-worked grafts show a disposition for the top cion to dominate the root system of the rootstock, whether an intermediate stem-piece of some other variety has been introduced or not. They show also a close correlation between vigor of cion and amount of root development: and a disposition for the least vigorous grafts to have roots which develop at an acute angle with the vertical.
Photographs taken at the completion of three growing seasons show that under the conditions of this experiment the top cion has had in some instances an appreciable influence upon the growth and character of the rootstock and of the grafted plant, and also that the intermediate stem-piece has had in other instances an effect upon the entire plant. Two major factors have seemed to dominate the situation, namely, the nature of the top cion and the compatibility of the union.
In combinations in which the graft unions appear smooth and congenial so that the union in itself is not a limiting factor, it has been the top cion which has dominated the shape and character of the rootstock, and not the intermediate stem-piece. Experiences in the literature show that the foliage developed by the cion may have an important hearing upon this situation. In double-working experiments in which foliage is permitted to develop upon the intermediate stem-piece, the question is raised as to what effect such foliage may have upon the rootstock.
While the intermediate stem-piece has seemed not in itself to have affected the root directly under the conditions of this experiment, yet it has in some instances affected it indirectly thru a decided effect upon the entire plant, as by dwarfing it.
Where a dwarf stock has been used, either as rootstock, intermediate stem-piece, or cion, the effect has been to dwarf the entire plant altho not always to the same degree for each position.
In seeking an explanation as to how the intermediate stem-piece may affect the plant, particularly with reference to dwarfing stocks, attention is immediately focused upon the graft union. What is known anatomically and structurally concerning the nature of the union between stock and cion supports this view. There is much evidence which points to the nature of the "union", or rather "the degree of union", as a very important consideration in stock and cion relation. In the consideration of the "stem" effect in double-worked trees, the authors wish to emphasize the region of contact between combining tissues, namely, the "union", rather than the "stem" itself, as the more important consideration.
Observations of stem-budded nursery trees agree with the findings with grafted trees in this experiment, namely, that when the union is compatible, it is the top cion and not the intermediate stem-piece which affects the root system. Height of budding has seemed not to influence root development except as height of budding introduces such problems as winter injury and physiological balance incident to pruning.
Differences in results from different sources, principally England and America, may be explained in part by differences in plant materials and in climatic factors. Rootstocks employed by American investigators have been preponderantly seedling in origin and selected by the exigencies of commercial competition for their vigor and congeniality with American varieties of fruit trees. Quite by contrast, many of the rootstocks used by English workers have been vegetative selections, having been perpetuated for some particular effect which they may have shown to exert upon the cion.
The question is no longer which one part dominates the entire plant, since all have been shown to have marked influence at one time or another. The rootstock may be the limiting factor one time, the union another, and the top cion another. The question is rather one of relation and degree between the consorting parts. The combination and intergrading of these factors determine the final result and give the "build-up" of a fruit plant.