Pollination in Orchards
by S.W. Fletcher
Cornell University Agricultural Experiment Station Bulletin 181 — March, 1900
I. Various reasons why flowers do not set.
All observing fruit-growers have seen trees which blossom full but do not set a fair amount of fruit; many have found their orchards unprofitable for this reason. It is a practical point to know the causes of this loss and the best way to prevent it.
Not all the flowers can set fruit.
In the first place, but a small percentage of the blossoms set fruit anyway, even in the most favorable seasons and with the most productive varieties. In blossoming time a Japanese plum tree is a mass of white, carrying scores of flowers on a single branch; yet scarcely a dozen fruits may set on that twig, and some of those must be removed or the tree will overbear. In the pollination work at Ithaca in 1899, 4,725 untouched blossoms, including apples, pears, plums and apricots, set but 617 fruits. The blossoms counted were those on the tree at large and were used for comparison with the hand crosses. This is about one fruit for every eight blossoms; yet most of the trees set what would be called a good crop. All of these blossoms were apparently uninjured by the winter, and the weather during the blossoming season was very favorable for the setting of fruit.
This normal failure in the setting of fruit blossoms may be due to a number of causes; as poorly nourished fruit-buds, lack of pollination, or winter injury to the pistils which cannot be seen with the eye alone. It is usually a distinct advantage to the fruit-grower, as it saves thinning. If all plum blossoms set fruit, the expense of thinning would be multiplied many times. Only when the failure of fruit blossoms to set becomes general, does the fruit-grower feel the loss and call for an explanation.
This wholesale failure in the setting of fruit is often called self-sterility. Properly speaking, a self-sterile tree is one which is self-unfruitful; it must have other varieties near it in order to bear well. But it appears that self-sterility in orchard fruits is often confused with the unfruitfulness resulting from other causes.
It would therefore be well to clear away this confusion at the outset, in order that the discussion of self-sterility may be better understood. The influences which sometimes make trees unfruitful, which are often confused with the unfruitfulness resulting from self-sterility, are (1) heavy wood growth, (2) the attack of fungi on the blossoms, (3) frosts, (4) unfavorable weather during the blooming season. It should also be said that a tree is not self-sterile when it does not blossom. This bulletin does not attempt to tell why trees do not bloom, except that it is generally due to poor management. The only thing which concerns us now is, why trees which blossom full do not set a reasonable amount of fruit.
Blossoms may drop because of heavy wood growth.
Young trees generally set little or no fruit the first few years, when they are growing fast, although they may blossom full. With most varieties this early dropping of the blossoms occurs only two or three seasons, but Northern Spy and a few other varieties of apples are often unfruitful ten to thirteen years from this cause. Older trees may show the same results if stimulated too highly with nitrogenous fertilizers. The logical remedy is to check this excessive growth of wood by withholding nitrogen or by putting the orchard into sod for a few years.
The direct cause of this unfruitfulness is not known. The stamens and pistils are usually well developed and pollen may be produced in abundance. Since young trees drop their blossoms as badly in a mixed orchard where other pollen is available, as when alone, the trouble probably lies more with the pistils than with the pollen.
Up to this limit of excessive growth, there is a fairly constant relation between the vigor of a tree and its productiveness. Lack of vigor causes much more unfruitfulness than excessive vigor. If a tree is unhealthy or dying because of poor nourishment, few of its blossoms are strong enough to set fruit. The same results may follow if the tree is exhausted by over-bearing.
Blossoms may be killed by fungi.
If the weather is warm and wet in early spring, conditions are favorable for the growth of fungi and it sometimes happens that fruit blossoms are “blasted” by the early growth of these parasites. The common brown-rot fungus often kills peach blossoms and may seriously decrease the setting of fruit. It is probable that this fungus sometimes attacks plum and cherry blossoms also. Apple and pear scab may kill the blossoms, but more often it kills the young fruits soon after they are set. Wherever spraying is practiced faithfully, the killing of fruit blossoms by fungi need not occur, especially if one thorough application is made to the trees before the buds open. The killing of pear blossoms by blight, however, cannot be prevented by spraying. The blossoms on Kieffer and LeConte trees are especially liable to be destroyed by the growth of blight microbes, which are carried from flower to flower. The only way to prevent this loss is to have no blighted trees in or near the orchard.
Winter and spring frost may injure the blossoms.
The unfruitfulness arising from winter or spring frost injury is sometimes confused with self-sterility. Various forms of winter injury to fruit buds are shown in Figs. 72-77. At A in Fig. 72 is a fruit bud which has been completely winter-killed and has made no growth whatever. B and C are buds which will never be able to open; while D is a very weak blossom which cannot set fruit. The single open flower on this branch is the only one which can possibly set fruit. A winter-injured cluster of Bietigheimer blossoms is seen in Fig. 73, with a section of one bud in Fig. 74 to show the shriveled stamens and pistils. The leaves in this cluster came through all right, but the flowers were injured. The single flower which has expanded is too small and weak to develop into fruit. These winter-injured clusters were common on all varieties of pears, particularly Angouleme and Manning Elizabeth, and on some varieties of apples, in the spring of 1899.
Two forms of winter or spring frost injury to the pistils are seen in Fig. 75 and 76, with a normal blossom for comparison in Fig. 77. A common form of injury is that in Fig. 75, in which the pistil is blackened and stunted, having made no perceptible growth during the opening of the flower. These pistils always drop from the tree soon after the petals have fallen. Another and not less common form of injury is that in Fig. 76, in which the pistil has made a partial growth but has no well developed ovary. Unless a careful examination is made, blossoms like this would not be considered as winter-injured. Of fifty which were tagged, none gave fruit, although several fruits grew to the size of peas. The killing of the pistils is the most common form of winter injury to fruit buds. Some of the native and Japanese plums had as high as 80 percent of defective pistils last spring, but with their enormous amount of bloom this did not materially decrease the crop of fruit which the trees were able to carry. The Japanese plums bloom so early that their blossoms are liable to be injured by frost in the middle states and south.
It is thus seen that the injury to fruit blossoms from cold is of all degrees. During the opening of a normal flower, the pistil grows. It is often taken for granted that if this growth occurs the pistil is uninjured; but it may be that even though a pistil reaches its full size, it may yet be so injured that it cannot develop into fruit. In 1899 about ten percent of the blossom buds of a Royal apricot opened fully, like the one in Fig. 72. All of these blossoms appeared to be perfect, with long pistils, plump ovaries and well developed stamens. Yet hardly a dozen fruits set on the whole tree, although the weather during the blooming season was ideal, bees were numerous, and some of the flowers were even crossed by hand with the pollen of other varieties. Since the variety had already shown itself so susceptible to winter injury, it is probable that this wholesale failure was due to the weakened vitality of the pistils, which could not be seen with the eye alone.
Some of the imperfect development, of flowers which we attribute to winter injury may be caused by unfavorable conditions during the previous season, when the buds were being formed; yet it seems likely that winter injury to pistils is more common and more serious than appears at first sight. These remarks on winter injury are introduced simply to emphasize the fact that all blossoms which do not set fruit are not self-sterile; and also to promote a more careful discrimination between the various causes which decrease the setting of fruit.
Rain may injure fruit blossoms.
The unfruitfulness which often follows a rain during the blooming season is sometimes confused with self-sterility. A careful fruit-grower watches the weather anxiously when his trees are in blossom, for he knows this is the most critical period in the growth of the crop. Injury to fruit blossoms from rain is common wherever fruit is grown, but is particularly serious along the Pacific coast and near the shores of the great Lakes. It has been estimated that more fruit is lost in California from cold rains during blooming time than from all other causes combined. Like winter injury to fruit buds, there is no way of preventing this loss except to secure a more favorable location, since it is not in man’s power to prevent rain, however much he may be able to induce it by bombarding the sky. Nevertheless, it is interesting to know in what way rain decreases the setting of fruit.
If a rain comes while the trees are in full bloom the pollen is washed from those anthers which have already opened, and is thus prevented from reaching the stigma. Should the rain be a short one, no serious harm need result from this loss of pollen, for the unopened anthers will burst and pollination will begin again soon after the sun comes out. The washing away of pollen has very little influence in decreasing the setting of fruit, particularly when the rain is short. There will generally be enough pollen to supply the pistils before or after the rain.
The poor setting of fruit which often follows a long rain and sometimes a shower is due more to a loss of vitality in the pollen or to some mechanical injury to the pistils; also, in large measure, to the fact that bees and other insects which promote the beneficial cross-pollination between varieties are absent. If the rain lasts for several days, the pollen may lose its vitality. After a week of rainy weather at Ithaca in the spring of 1898, nearly all the pollen of the apricots then in bloom was disorganized and stuck together, so that it could not possibly grow and fertilize the pistils. Some of this pollen is shown in Fig. 78. It is also natural to suppose that a hard rain may wash off, dilute, or otherwise injure the juices of the stigma so that the pollen cannot germinate after it falls upon the stigma. Perhaps a long “spell” of wet weather may even kill the pistils after they have been fertilized.
Thus a rain during the blooming season may decrease the setting of fruit in four ways: (1) By preventing the pollen from reaching the stigma, both because it is too wet to fly and because pollen-carrying insects are absent. This is important only when the rain lasts several days and most of the pistils pass their receptive state before the rain ceases. (2) By destroying the vitality of the pollen. (3) By injuring the stigma. (4) By preventing fertilization or the germination of the pollen because of low temperature.
The blossoms may be injured by strong or drying winds.
Near the sea and large lakes, fruit blossoms may be whipped off by very severe winds. In such cases a mixed windbreak of deciduous and evergreen trees may be used to advantage. Drying winds during the blossoming season are not common in the east but are often serious in some parts of the west. Luther Burbank, one of our best observers and experimenters in orchard pollination, says a dry wind sometimes causes a short fruit crop in some parts of California by drying up the juices of the stigma so that the pollen cannot germinate.
There have been described in the preceding pages some of the influences which decrease the setting of fruit. These were mentioned only to make more clear a talk about self-sterility, an influence which is second only to the winter injury of fruit buds in the loss caused to the commercial fruit grower. Since the loss from unfavorable weather usually cannot be prevented, while the loss from self-sterility can in a large measure, the latter subject deserves more than the brief notice which has been given to the former at this time.
There are some trees which blossom full year after year but set little or no fruit, even in the most favorable seasons. These trees are usually in solid blocks, or at a distance from any other variety. Planting near them other trees of the same variety does not make them fruitful; but if trees of other varieties are planted near they are often made fruitful. A self-sterile variety is one which is unable to set fruit when alone; in order to be productive, it must be planted near some other variety. Two varieties very commonly self-sterile are Miner and Wild Goose plums. Large blocks of the Kieffer pear and some of the great prune orchards on our Pacific coast have been unprofitable from this cause. Besides these striking examples, there is reason for believing that much of the unsatisfactory fruiting of orchards all over the country is due to the isolation or indiscriminate mixing of varieties.
The main cause of self-sterility.
In general, the cause of self-sterility is that the pollen of a variety is unable to fertilize the pistils of that same variety. That is, if pollen from a Wild Goose blossom falls on a Wild Goose pistil, whether on the same tree or any other Wild Goose tree, no fruit will result as a rule. The pollen of a self-sterile variety may be and generally is produced in abundance and is well formed. Wild Goose generally bears pollen freely, although it is one of the most self-sterile varieties in cultivation. The Bartlett pear is often self-sterile, yet its pollen is perfect (Fig. 80). The pollen of a self-sterile variety also has vitality, for it will fertilize the pistils of other varieties. For example, plant together trees of the two self-sterile varieties, Miner and Wild Goose, and both will often be made fruitful, because the pollen of each, though infertile on itself, is fertile on the other. It is not known in what way this infertility is usually shown, but with Wild Goose at least, the pollen grain actually germinates and the pollen tube passes down to the ovule. Why the two sexes are unable to unite after having got thus far, the embryologist has not yet told us.
Minor causes of self-sterility.
Aside from the impotency of pollen, the main cause of self-sterility, there may be several other incidental causes. Goff and Waugh have shown that self-sterile varieties of native plums often have a large percent of pistils which are too weak to develop into fruit. This could not be a general cause of self-sterility, however, for self-sterile varieties can usually be made fruitful by planting other varieties near them. This shows that there are enough sound pistils on the tree for a good crop of fruit, provided they receive the right kind of pollen.
Again, the blossoms of some varieties may produce but a small amount of pollen. When these varieties are planted alone they may not have enough pollen to set a good crop, even though the pollen is fertile on its associated pistils. The amount of pollen which flowers produce is greatly modified by weather conditions and the vigor of the tree.
Many plums are worked on Marianna and Miner stocks, two of the most self-sterile varieties in common cultivation. It has been thought that possibly there might be an influence of the stock on the scion in the direction of self-sterility, but this assumption seems to be without foundation.
Finally, the stamens and pistils of a tree may not mature simultaneously, which would make a tree unfruitful unless pollen is supplied from other sources. With many varieties of orchard fruits the pistil of each flower matures a little before the stamens; and not infrequently the stamens mature before the pistil is ready to receive the pollen. But there is usually enough variation in the opening of flowers on the same tree to promote pollination with each other and so prevent serious loss from this alternate ripening of the sexes. Defective pistils, scanty pollen supply, and the premature ripening of either pistils or stamens may often be important in determining the fruitfulness of a tree; but the main cause of unfruitfulness in most self-sterile varieties is the failure of the pollen to fertilize its associated pistils. This cause cannot be removed, but its injurious results may often be prevented by a judicious selection of varieties.
A practical application.
The practical bearing of the self-sterility problem is this: There are certain varieties of fruit which we wish to grow largely for the general market, but we find that they are not productive when planted alone. They need the pollen of other varieties to make them fruitful. Then we must do what some of our most intelligent fruit-growers have been doing for years — plant other varieties near them as pollinizers. Orchardists along the Atlantic coast have been obliged to do this with Kieffer. The Californians often find it necessary with their prunes; and many an unproductive orchard of Wild Goose has been made fruitful by being partially top-worked with another variety. Cross-pollination of varieties is no longer a theory; it is an established orchard practice.
The history of the self-sterility discussion.
There are at least sixty species of plants which are known to be often sterile with their own pollen. The study of this problem had its origin mainly in the investigations of Darwin. While Darwin was not the first to observe the value of cross-pollination, he so far exceeded his predecessors in this, as in most other work, that the beginning of a systematic study of self-sterility is usually dated from the publication of his “Origin of Species” in 1859.
The unfruitfulness arising from self-sterility had been noticed many years before by fruit growers. The benefit which some varieties gained by being planted near other varieties also had been noticed, and mixed planting was often practiced with success, particularly with Wild Goose and Miner. There are now one hundred and twenty-six entries in my bibliography of references to “barren” trees in American literature before the appearance of Waite’s bulletin in 1894. The real cause of this barrenness, however, was not known definitely before the experiments of Waite; although it had long been supposed by many to be the pollen. Of late years, many experimenters have done careful work along this line. Among these are Goff, Waugh, Craig, Kerr, Crandall and Heideman on orchard fruits; Beach, Earle, T. V. Munson, Whitten, and Green on grapes. The California and Oregon State Boards of Horticulture are also making a special inquiry on the self-sterility of prunes.
Varieties which are often self-sterile.
Self-sterility is not a constant character with any variety. It is influenced by the conditions under which the tree is grown, as are the size, shape and color of the fruit. The adaptation of a variety to soil and climate has much to do with its self-fertility, and if a tree is poorly nourished it is more likely to be infertile with its own pollen. No one can separate varieties of fruit into two definite classes, the self-sterile and the self-fertile. Thus Bartlett and Kieffer are often self-sterile, but there are orchards of both which are self-fertile. The same may be said of many other varieties. The best that can be done, therefore, is to give a list of those varieties which tend to be more or less self-sterile and which it would be unsafe to plant alone.
Following is a conservative list of these risky varieties, drawn both from experimental work and from the reports of over five hundred fruit growers who have favored me with their experience. Pears: Angouleme (Duchess), Bartlett, Clapp, Idaho, Kieffer, Nelis. Apples: Bellflower, Primate, Spitzenburg, Willow Twig, Winesap. Plums: Coe Golden Drop, French Prune, Italian Prune, Kelsey, Marianna, Miner, Ogon, Peach, Satsuma, Wild Goose, and according to Waugh and Kerr, all other varieties of native plums except Robinson. Peach: Susquehanna. Apricot: White Nicholas. Cherries: Napoleon, Belle de Choisy, Reine Hortense. Most of these varieties are self-fertile in some places, but the weight of evidence shows them to be uncertain.
It must not be inferred that all other varieties are always able to set fruit when planted alone. There are some, however, which have exceptionally good records for fruitfulness when planted in solid blocks, other conditions being favorable. Among these are: Apples: Baldwin, Ben Davis, Fallawater, Janet, Oldenburg, Rhode Island Greening, Red Astrachan, Smith Cider. Plums: Burbank, Bradshaw, DeSoto, Green Gage, Lombard, Robinson and some of the common blue Damsons.
All this goes to show that the problem of self-sterility is as much a study of conditions as of varieties. We can set no limits; we can only indicate tendencies.
The great and growing Kieffer pear industry in the eastern United States warrants a fuller discussion of this variety. Many large blocks of Kieffer are being planted with no other varieties intermingled, and it is an important point to know whether this practice will give the best results. Eight blocks of Kieffer in New Jersey and Delaware have been reported as completely or partially unfruitful because of self-sterility, and there are also many solid blocks of Kieffers in the same states which bear well. Kieffer is unreliable, especially on the Delaware peninsula. A large block of Kieffer may be productive, but it does not pay to take the risk, particularly since the pollen of other varieties is likely to give better fruit, as will be seen later on.
Selecting the pollinizer.
Let us suppose that we intend to plant a large block of an uncertain variety, as Kieffer, because it has distinct merit as a market sort. We wish to plant with it some other variety to make it fruitful. There are two points to be considered when selecting a pollinizer for Kieffer or for any other self-sterile variety; the choice should not be indiscriminate. These are simultaneous blooming, and mutual affinity.
The first and most important point is that the two shall blossom together, since the only way in which a pollinizer can make a self-sterile variety fruitful is by supplying it with pollen. This means that the pistils of the self-sterile variety must be receptive when the stamens of the pollinizer are ripe, which is possible only with simultaneous blooming.
The comparative blooming of varieties is more or less a local problem. Differences of latitude, altitude, soil, nearness to large bodies of water, and weather conditions during the blooming season not only hasten or retard the time of blooming but also disturb the order in which the different varieties open. Varieties blossoming together at one place may not at another. The best that can be done in the way of generalizing on the question of simultaneous blooming for cross-pollination is to make a chart for each well marked geographical district. This will indicate in a general way which of the standard commercial varieties may be expected to bloom together; each fruit grower should be prepared to make minor corrections for his own farm. Until more definite knowledge is available, each orchardist should learn how varieties bloom in his own neighborhood before planting them for cross-pollination. It is better, but not always necessary, that the two should bloom exactly together; if they overlap two or three days that is often enough.
It is sometimes desirable to plant varieties of different botanical species together for cross-pollination, but this will often be impracticable because of the difference in their blooming seasons. Thus the Oriental pears, as Kieffer, and the European pears, as Bartlett, usually do not blossom together. Kieffer generally blooms several days before Bartlett, hence it is necessary to pollinate it with a variety of its own class, as Le Conte or Garber. In some places, however, the two groups blossom approximately together, and then varieties like Bartlett and Seckel should be used in preference to Le Conte or Garber, since their fruit has a greater market value and the trees are less likely to blight. Whenever the European pears are used as pollinizers for Kieffer it would be well, if otherwise practicable, to work them on quince roots. Standard Kieffers will often bloom two or three years before standard Bartletts planted at the same time, and unless early blooming dwarfs are intermingled they may be unproductive these first few years.
The three classes of commercial plums, — Japanese, domestica and native, — will usually bloom at different periods in the order named; but when a “spell” of warm weather succeeds a cold and backward spring, varieties of all these groups will come on nearly together and cross-pollination will result. In some places the blooming seasons of these groups overlap so that some varieties of each might be used regularly for cross-pollination.
The mutual affinity of varieties.
Another point to be looked after when selecting a pollinizer for Kieffer, or for any other self-sterile variety, is the mutual affinity of the two. That is, will the pollen of the pollinizer fertilize the pistils of the self-sterile variety readily and also develop them into high grade fruit? At present but little is known about this matter. Taking first the possibility of cross-pollination between varieties of different species, there seems to be no doubt but that many varieties of native, Japanese and domestic plums will fertilize each other. Orchard experience in many places indicates this; as when Satsuma is used to pollinate Coe Golden Drop in Californian prune orchards. Several successful crosses between the three were also made at Ithaca the past season. Among these are Abundance X Grand Duke (Fig. 84), Georgeson X Wayland, Berckmans X Coe Golden Drop, Coe Golden Drop X Satsuma. That is, if we wish to use Satsuma as a pollinizer for Coe Golden Drop, or Lombard for Wild Goose, the probability is that the combination would work, if the two varieties bloom together; but since the three groups usually bloom at somewhat different periods there can be no general cross-pollination outside the limits of the species.
Numerous crosses and common orchard practice have also shown that the European pears, as Bartlett, and the Sand Pear hybrids, as Kieffer, will fertilize each other regularly when they bloom together. Several Kieffer fruits from Bartlett pollen and Bartlett fruits from Kieffer pollen were secured in the crossing work of 1899. In fact, my experience has been that if Kieffer pollen is put on the pistils of our common pears, of the European class, it will usually produce larger fruit than pollen from most varieties of that type. Kieffer is a good pollinizer for Bartlett, Angouleme, Clapp, Nelis and like varieties, when they bloom together. In Fig. 81, compare the size of the Seckels which received Kieffer pollen with those which had Lawrence pollen. The specimens shown are typical of thirty fruits secured from these two crosses in 1899.
It is necessary to study not only the mutual affinity of varieties belonging to different species, but also of varieties of the same species. Some varieties will not fertilize each other, though blossoming at the same time. Kerr has found that Whitaker plum will not fertilize Wild Goose nor will Early Red help Caddo Chief. Again, the pollen of some varieties will give better fruit than that of others when used on the pistils of self-sterile or even on self-fertile varieties. There is very little definite knowledge as to what varieties are best adapted for pollinating self-sterile sorts. Waugh and Kerr have studied this point with native plums for several years and their judgment is united in a table of recommended pollinizers for plums. A few results from crosses made at Ithaca in 1899 will illustrate this point. Fig. 81 shows the comparative size of Seckel when pollinated with Kieffer and with Lawrence pollen. Clapp pollinated with Kieffer was also larger than Clapp pollinated with Lawrence or Louise Bonne. Howell blossoms which received the pollen of Clapp gave fruits of nearly twice the size of those which received Bartlett pollen. Bartletts crossed with Angouleme were larger than Bartletts crossed with Sheldon. In some cases no difference could be noticed, yet most of our standard commercial varieties will be likely to yield enough better fruit when planted with some varieties than with others, to make a study of this point worth the while.
Some of the combinations which have been very successful in the commercial orchards of the country are: Bartlett with Nelis, Flemish Beauty, Easter, White Doyenne; Idaho with Bartlett; Kieffer with Le Conte, Garber; Coe Golden Drop with French Prune, Green Gage, Italian Prune (Fellenburg); Satsuma with Abundance, Burbank, Red June; Miner with De Soto, Forest Rose, Wild Goose; Wild Goose with De Soto, Newman, Miner.
Does crossing change the appearance of the fruit?
In connection with the mutual affinity of varieties which are selected for cross-pollination, there comes the question of the “immediate influence” of pollen. For instance if Seckel pollen is put on Kieffer pistils, will it impart the Seckel flavor, color and characteristic shape to the resulting fruit? Of course the characters of both may be united in the seeds, and the trees which come from these seeds may be expected to be intermediates; but is the flesh of the fruit ever changed by foreign pollen?
The increase in size which often follows crossing cannot be called a true immediate influence, for the foreign pollen generally stimulates the fruit to a better growth because it is more acceptable to the pistils, not because it carries over the size-character of the variety from which it came. In 1899, Hyslop Crab pistils which were fertilized with pollen from the great Tomkins County King, grew into fruits of the usual crab size. An immediate influence in size may be possible, for the size of the fruit is nearly as constant a varietal character as is the shape; but most of the increased size in crosses of orchard fruits probably arises from the fact that the pollen is more acceptable.
Setting aside the usual gain in size resulting from crossing, we wish to know whether there will be any change in the shape, color, quality and season of ripening of the fruit. A few undoubted instances of this influence have been noticed with some plants in which the seed is the principal part of the fruit, as the mixing of sweet corn and field corn; also perhaps in various peas and beans. When the seed is surrounded by a fleshy pulp, however, as in our common orchard fruits, it is still in dispute whether this pulp is influenced, however much the seeds themselves may be. Most have formed their convictions about the immediate influence of pollen from observation, rather than from experimental proof. It does not necessarily follow that “sweet and sour” apples are due to cross-pollination, nor that the russet on Greening apples borne on the side of the tree next to a Roxbury was produced by the influence of the Roxbury pollen.
Most of the changes in fruit which are attributed to the influence of cross-pollination are due to variation. Every bud on a tree is different in some way from every other bud on that tree and may develop unusual characters, independent of all the other buds, according to the conditions under which it grows.
The best way to determine whether there is an immediate influence of pollen is by hand crossing. Among the forty-five different crosses which were made in 1899 with this particular point in view, not one showed any change which could be positively attributed to the influence of pollen. Even the concentrated sweetness of Seckel made no impression on the poor quality of Kieffer; nor were there any constant differences in color, shape, or season of ripening in any of the other crosses. Nearly everybody who has crossed varieties of orchard fruits has had a similar experience.
Most of the evidence supporting the theory that there is an immediate influence of pollen in the crosses of fruits comes from observation; most of the evidence against it comes from experiment. The observer, however careful, is likely to jump at conclusions; the experimenter tries to give due weight to every influence which might bear on the problem. Since many observers and a few experimenters have found what seems to be an immediate influence of pollen on the fruit, we cannot doubt but that this influence is sometimes exerted. But it is certainly much less frequent than is commonly supposed.
The distribution of the pollinizers.
Having selected a pollinizer with reference to simultaneous blooming and mutual affinity, the fruit-grower now wishes to know how many trees will be necessary to pollinate the self-sterile variety. There are three things to be considered here: the ability of the pollinizer to produce pollen, its market value, and the class of fruit to which the self-sterile variety belongs.
Varieties differ in the amount of pollen which they produce, and the pollen production of the same variety is also greatly modified by differences in locality and season. Other things being equal, the variety which produces pollen freely could be used more sparingly in a block of self-sterile trees than one of the scanty pollen production. Little comparative observation has been made on this point as yet; but as a matter of fact, most of our common varieties produce an abundance of pollen.
The number of trees of the pollinizer would also depend largely on whether it has value in itself. If we are planting Le Conte to pollinate Kieffer, we would naturally try to get along with the least possible number which will do the work; but if Bartletts are to be used for the same purpose, we can afford to increase the proportion. Some growers plant every tenth row to the pollinizer, but the proportion should usually be greater. This might be enough if the weather during the blossoming season is very favorable for cross-pollination by wind and insects; but if it is showery, the pollinizers should be more abundant, in order that cross-pollination may be more general during the bright weather between showers. If using Garber or Le Conte to pollinate Kieffer, every third row may be the pollinizer; if using Bartlett, every other row. For apples, cherries and domestic or Japanese plums, the same proportion may be used. In a commercial orchard, the pollinizer should be planted in a solid row. Theoretically, it is much better to have the pollinizer more evenly distributed among the self-sterile trees; practically, it will not pay to so mix them except in small orchards.
The advantages of general mixed planting.
It would appear that the only thing to do now is to find out what varieties are inclined to be self-sterile and the varieties which are best adapted for fertilizing them. But as a matter of fact, cross-pollination gives better results with nearly all varieties, be they self-sterile or self-fertile. A variety may be able to bear good fruit when it is planted alone, but it will often bear better fruit if suitable varieties are near it. Mixed orchards are more productive than solid blocks, taking the country over. It is a common observation in Western New York that Baldwins in mixed orchards are more uniformly productive than Baldwins in large blocks. Furthermore, although a variety may be able to set an abundance of fruit with its own pollen, this fruit will often be smaller than if other pollen were supplied. From a number of experiments made in 1899, a few representative results are here given to illustrate this point.
Compare the size of self-pollinated and cross-pollinated fruits in Figs. 83-85. In some varieties the difference was very marked, as with Stark and Longfield apples (Fig. 83); in others the difference was not so marked, as Abundance (Fig. 84); while a few showed no appreciable increase in size from cross-pollination, as Talman Sweet (Fig. 85) and Bradshaw. The difference between the cross- and self-pollinated Starks and Longfields is so striking that one would almost be tempted to think the self-pollinated fruits were wormy, but they were not. The self-pollinated Talmans and Bradshaws were apparently as fine in every way as the cross-pollinated fruits. Manning Elizabeth pear also was not benefited by pollen from other varieties.
The three self-pollinated Longfields here shown (Fig. 83) had but five sound seeds; while the two crossed specimens had seventeen sound seeds. In general, cross-pollinated fruits have more good seeds than self-pollinated fruits, but there is no constant relation between the size of a fruit and the number of seeds it contains. Some of the biggest apples or pears may have only two or three good seeds. In case the ovules in one cell of an apple or pear core are not fertilized, that part of the fruit adjoining is often stunted and the fruit becomes lop-sided in consequence; but this, likewise, does not always follow.
All of the above varieties are self-fertile. They will produce fruit with their own pollen. But we have seen that some of them will produce better fruit if other pollen is supplied. Is it not worth while, then, to plant pollinizers even with self-fertile varieties — that is, to practice mixed planting with all varieties? There are many good reasons for doing this.