THE GENUS PRUNUS

The great genus Prunus includes plums, cherries, almonds, apricots, peaches, and the evergreen cherries or cherry laurels. Its widely distributed species number a hundred or more for the world, nearly all of which belong north of the equator. The species of the genus are widely distributed in both the eastern and western hemispheres, the flora of eastern America and of western Asia being especially rich in species and individuals. For most part the species of Prunus belong to the Temperate Zone, but several of the evergreen cherries, usually grouped in a section under Laurocerasus, are found in the tropics and sub-tropics.

The species cultivated for their edible fruits are found only in the Temperate Zone of the Northern Hemisphere. Of these the peach and the almond are believed to have come from eastern and southeastern Asia; the apricot is thought to be a native of northern China; the wild forms of the cultivated cherries are Eurasian plants, very generally distributed in the regions to the northward where the two continents meet. The habitats of the cultivated plums are given in detail in the text that follows, as Asia, Europe and America. Presumably the genus had its origin in some of the above regions; but where the center is from which the species radiated can never be known. Indeed, with present knowledge it cannot be said in what region Prunus has most species, is most productive of individuals, or shows highest development and greatest variability,—facts which might give some evidence as to the origin of the genus. It is probable that the greatest number of combinations of the above evidences can be shown for Asia and more especially for the Eurasian region, where Europe and Asia meet; yet North America has two score or more indigenous species about half of which are arborescent.

The history of the genus Prunus is one of continual changes. Of the botanists who have done most toward classifying plants, Ray, Tournefort, Dillenius and Boerhaave, pre-Linnaean botanists, placed only the plum in Prunus. Linnaeus adopted the name used by his predecessors for the plum alone, for a genus in which he also placed plums and cherries. Adanson and Jussieu returned to the pre-Linnaean classification but Gaertner followed the grouping of Linnaeus. Necker, DeCandolle, Roemer and Decaisne held that the plum alone belongs in Prunus. Bentham & Hooker, Gray and his co-workers in the several revisions of his botany, and Engler & Prantl, great authorities of the Nineteenth Century, extend the genus to include all of the stone-fruits. On the other hand, Britton and Brown, in their recent flora of northern United States and of Canada restrict the group to plums and cherries. Horticulturists have been less divided in their opinions than the botanists and have very generally placed all of the stone-fruits in one genus. The diversity of views as to what plants belong in Prunus, indicated above, suggests that the differences separating the several stone-fruits may not be many nor very distinct. This is true, and makes necessary a discussion of the characters which distinguish these fruits.

The flowers of true plums are borne on stems in fascicled umbels and appear either before the leaves or with or after them. Flowers of the cultivated cherries are similarly borne, though the fascicles are corymbose rather than umbelliferous. But apricot, peach and almond flowers are stemless or nearly so and solitary or borne in pairs appearing before the leaves.

The fruits of plums and cherries are globular or oblong, fleshy, very juicy, with smooth or slightly hairy skins. Peaches, apricots and almonds are more sulcate or grooved than plums and cherries and the first two have juicy flesh, but that of the almond is dry and hard or skin-like, splitting at maturity thereby liberating the stone; these last three fruits are distinguished from plums and cherries by having very pubescent or velvety skins though rarely, as in the nectarine, a botanical variety of the peach, and in a few cultivated apricots, the skins are smooth.

The stone of the plum is usually compressed, longer than broad, smooth or roughened, thickish and with an acute margin along the ventral suture and thinnish or grooved on the dorsal suture. The stone of the cherry is usually globular, always much thickened, smooth or a very little roughened, ridged and grooved on the ventral suture, with a thin, scarcely raised sharp margin on the dorsal suture. The stone of the apricot is similar to that of the plum though thicker walled, with a more conspicuous winged margin, and is sometimes pitted. The stone of the peach is compressed, usually with very thick walls, much roughened and deeply pitted. In the almond the stone resembles in general characters the peach-stone, but all almond shells are more or less porous and often fibrous on the inner surfaces. The stone is the part for which the almond is cultivated and is most variable, the chief differences being that some have thick hard shells and others thin soft shells.

The leaves of plums are convolute, or rolled up, in the bud. Cherry, peach and almond leaves are conduplicate, that is are folded lengthwise along the midrib in bud while the leaves of the apricot, like those of the plum, are convolute. The manner in which the leaves are packed in the bud is a fine mark of distinction in stone-fruits. In size and shape of leaves, as well as in the finer marks of these organs, the botanist and pomologist find much to aid in distinguishing species and varieties but little that holds in separating the sub-genera. The last statement holds true with the floral organs also.

The near affinity of the stone-fruits is further shown by the fact that plums and apricots, plums and cherries, and the several species of each of the distinct fruits inter-hybridize without much difficulty. It is a fact well-known that hybrids often surpass their parents in vigor of plant and in productiveness and this has proved true with most of the hybrids in Prunus of which we have accounts, thereby giving promise of improved forms of these plants through hybridizing. The great variation in wild and cultivated native plums is possibly due to more or less remote hybridity.

Prunus is a most variable genus. This is indicated by the several sub-genera, the large number of species and the various arrangements of these groups by different authors. At their extremes sub-genera and species are very distinct, but outside of the normal types, and sometimes in several directions, there are often outstanding forms which establish well-graded connections with neighboring groups. For example, among the American plums there are but few species between which and some other there are not intermediate forms that make the two species difficult to distinguish under some conditions. There is also a wide range of variation within the species. The modifications within the species are oftentimes such as to change greatly the aspect of the plant; the trees may be dwarf or luxuriant, smooth or pubescent; may differ in branching habit, in leaf-form, in size and color of the flowers, in the time of opening of leaf and flower-buds, in color, shape, size, flesh, flavor and time of ripening of fruit, in the stone and in all such characters as climate and soil environment would be liable to modify.

This inherent variability is one of the strong assets of the genus as a cultivated group of plants, for it allows not only a great number of kinds of fruits and of species but a great number of varieties. Besides, it gives to the genus great adaptiveness to cultural environment, in accordance with climate, location, soil and the handling of the trees. The cultivator is able to modify, too, the characters of members of the genus to a high degree in the production of new forms, but few, if any, groups of plants having produced as many cultivated varieties as Prunus.

The genus Prunus is preeminent in horticulture, furnishing all of the so-called stone-fruits, fruits which for variety, delicious flavor and beauty of appearance, probably surpass those of any other genus, and which, fresh or dried, are most valuable human foods. The seeds of one of the fruits belonging to Prunus, the almond, are commercially important, both for direct consumption and for the oil which is pressed from them; in India a similar oil is obtained from the seeds of peaches and apricots, while in Europe an oil from the seeds of the Mahaleb cherry is used in making perfumes. Various cordials are made from the fruits of the several species, as kirschwasser and maraschino from cherries, zwetschenwasser and raki from plums, and peach brandy from the peach; while fruits and seeds of the several species are soaked in spirits for food, drink and medicinal purposes. The bitter astringent bark and leaves are more or less used in medicine as is also the gum secreted from the trunks of nearly all the species and which, known as cerisin, is used in various trades. The wood of all of the arborescent species is more or less valuable for lumber, for cabinet-making and other domestic purposes.

Prunus is prolific also in ornamental plants, having in common to recommend them, rapidity of growth, ease of culture, comparative freedom from pests, and great adaptability to soils and climates. The plants of this genus are valued as ornamentals both for their flowers and for their foliage. Many cultivated forms of several of the species have single or double flowers, or variegated, colored or otherwise abnormal leaves, while the genus is enlivened by the evergreen foliage of the cherry laurels. Nearly all of the plants of Prunus are spring-flowering but most of them are attractive later on in the foliage and many of them are very ornamental in fruit.

PLUMS.

Of all the stone-fruits plums furnish the greatest diversity of kinds. Varieties to the number of two thousand, from fifteen species, are now or have been under cultivation. These varieties give a greater range of flavor, aroma, texture, color, form and size, the qualities which gratify the senses and make fruits desirable, than any other of our orchard fruits. The trees, too, are diverse in structure, some of the plums being shrub-like plants with slender branches, while others are true trees with stout trunks and sturdy branches; some species have thin, delicate leaves and others coarse, heavy foliage. In geographical distribution both the wild and the cultivated plum encircle the globe in the North Temperate Zone, and the cultivated varieties are common inhabitants of the southern temperate region, the various plums being adapted to great differences in temperature, moisture and soil in the two zones.

The great variety of plums and the variability of the kinds, seemingly plastic in all characters, the general distribution of the fruit throughout the zone in which is carried on the greatest part of the world’s agriculture, and the adaptation of the several species and the many varieties, to topographical, soil and climatic changes, make this fruit not only one of much present importance but also one of great capacity for further development. Of the plums of the Old World the Domesticas, Insititias and probably the Trifloras have been cultivated for two thousand years or more, while the work of domesticating the wild species of America was only begun in the middle of the last century. There are about fifteen hundred varieties of the Old World plums listed in this work, and since the New World plums are quite as variable, as great a variety or greater, since there are more species, may be expected in America.

An attempt is made in The Plums of New York to review the plum flora of this continent, but the species considered fall far short of being all of the promising indigenous plums; not only are there more to be described, but it is probable that species here described will in some cases be sub-divided. The development of the pomological plum-wealth of North America is but begun. Not nearly as much has been done to develop the possibilities of the European plums in America as in the case of the other tree-fruits. Probably a greater percentage of the varieties of Old World plums commonly cultivated came from across the sea, than of the varieties of any other of the orchard-fruits which have been introduced. Much remains to be done in securing greater adaptability of foreign plums to American conditions. Native and foreign plums are also being hybridized with very great advantage to pomology.

The Plums of New York is written largely with the aim of furthering the development of plums in America, the possibilities of which are indicated in the preceding paragraph. With this end in view the first task is to name and discuss briefly the characters of plums whereby species and varieties are distinguished, with a statement, so far as present knowledge permits, of the variability of the different characters. It is absolutely essential that the plum-grower have knowledge, especially if he aspires to improve the fruit by breeding, of the characters of the plants with which he is to work. These are in the main as follows:

All species and some horticultural varieties have more or less characteristic trees. Making due allowance for environment—food, moisture and light—many plum groups can be readily distinguished by the general aspect of the plant. Of the gross characters of trees, size is usually most characteristic. A species, for example, is either shrubby or tree-like. Yet under varying environment, size of plant and of the parts of the plant, are probably the first to change. Habit of growth is nearly as important as size and varies but little under changing conditions. A species or variety may be upright, spreading, drooping or round-topped in growth; head open or dense; the tree rapid or slow-growing. Hardiness is a very important diagnostic character, plums being either hardy, half-hardy or tender. Both species and varieties respond in high degree to the test of hardiness, the range for varieties, of course, falling within that of the species. Productiveness, regularity of bearing, susceptibility to diseases and insects, and longevity of tree are all characters having value for species and varieties and with the exception of the first named, are little subject to variation.

The thickness, smoothness, color and manner of exfoliation of the outer bark and the color of the inner bark have considerable value in determining species but are little used in determining horticultural groups. It is well recognized that all plums have lighter colored bark in the South than in the North. The branches are very characteristic in several species. The length, thickness and rigidity of the branch and the length of its internodes should be considered, while the direction of the branch, whether straight or zigzag, are very valuable determining characters and relatively stable ones, seeming to change for most part only through long ranges of climatic conditions. So, too, the arming of a branch with spines or spurs and the structure of such organs are important. The color, smoothness, amount of pubescence, direction, length, thickness and the appearance of the lenticels, the presence of excrescences on the branchlets of the first and second year’s growth and the branching angle, are all worthy of consideration though quite too much has been made of these characters, especially of pubescence, in determining species, for they are all extremely variable.

The size, shape and color of leaf-buds and of their outer and inner scales and the margins of the scales differ in different species. Possibly the most evident, and therefore readiest means of identifying species, at least, is by the leaves. It is true that leaves are very variable but always within limits, and either individually or collectively in giving the general aspect to a tree they are characteristic. Modifications of leaves most often occur in very young plants, those growing in bright sunshine or deep shade and on sprouts or suckers, but none of these are usually sufficient to mislead as to species. Leaf-size and leaf-form are the first characters to be noted in determining a plum but these are closely followed in value by leaf-color, leaf-surface, leaf-thickness and leaf-margin. Leaf-size is variable, depending much upon the conditions noted above but leaf-form varies but little in the several species. So, too, the color of leaves is very constant throughout a species, for both surfaces, though impossible to describe accurately in words and very difficult to reproduce in color-printing. There is a marked difference in autumnal tints not only of species but of varieties but these are not very constant in any one location and must vary greatly under different environments. The thickness of the leaves of the several species is a distinctive character. Species of plums have very different leaf-surfaces as regards reticulation, rugoseness, pubescence and coriaceousness, all of these characters being quite constant, though it is to be noted that roughness of leaves and pubescence are increased by exposure to the sun and by the influence of some soils. There is, indeed, considerable variation in the pubescence of the leaves of all species of plums in different parts of the country and probably too much has been made of pubescence as a determining character.

The margins of leaves are very characteristic of species and scarcely vary under normal conditions if the teeth at the middle of the sides be taken rather than those toward the base or apex, these very often being crowded, reduced or wanting. The presence of glands, their position, size, shape and color, help to characterize several species and seem to be fairly constant guides. Some species and a great number of varieties have the distinguishing marks of gland-like prickles tipping the serrations in the leaf-margins. Length, thickness, rigidity and pubescence of petiole have some taxonomic value. Stipules usually offer no distinguishing marks other than those mentioned under leaves.

The blossoms of plums are very characteristic, giving in flowering time a distinctive aspect to all species and distinguishing some horticultural varieties. The flowers of all the species are borne in clusters, differing in number of individuals, according to the species; so, too, the flowers in the different species vary in size, color, in length of their peduncles, and in pubescence, especially of the calyx. Flower-characters are constant, taking them as a whole, yet there are some variations that must be noted. One of the most marked of these is in the time of appearance of the flowers; in the South they appear before the leaves but in the North with the leaves. On the grounds of this Station there are notable exceptions to the latter statement, with varieties of species showing considerable variation in this regard. There are some remarkable variations within species as regards size and color of the corolla and glands and pubescence of the calyx, depending upon the environment of the plant; but on the whole these characters are very constant. The fragrance of the flowers of plums varies from a delicate, agreeable odor to one that is quite disagreeable in some species as in Americana; the odor seems to be a constant character.

Of all structures of the plum the fruit is most variable, yet fruits are sufficiently distinct and constant, especially within species, to make their characters very valuable in classification. Species, whether wild or cultivated, may be distinguished in greater or less degree by the period of ripening of the fruits, though in this regard the cultivated varieties of the several species vary greatly and in the wild state trees of native plums in the same locality, even in the same clump, may vary in ripening as much as from two to four weeks. Species are distinguished by size, shape, color, flesh, flavor and pit among the grosser characters of the structure and by amount of bloom, stem, cavity, apex, suture and skin among the minor characters. The fruit is usually the first part of the plant to respond to changed conditions.

Characters derived from seed structures are generally accounted of much value by botanists in determining species. Such is the case with plums. This Station has a collection of stones of over three hundred cultivated varieties of plums and some specimens of nearly all the different species. The stones illustrated in the color-plates in this book show that this structure is quite variable in size, shape, in the ends, surfaces, grooves and ridges, even within a species; nevertheless in describing the several hundred forms of plums for The Plums of New York the stone has been quite as satisfactory, if not the most satisfactory, of any of the organs of this plant for distinguishing the various species and varieties.

The reproductive organs of plums afford several characters and would seem to offer means of distinguishing botanical and horticultural groups, but they are so variable in both cultivated and wild plants as to be very misleading. Not only do these organs differ very often in structure but also in ability to perform their functions. Bailey[1] has called attention to the remarkable self-sterility of some varieties of the native species of plums, due to the impotency of the pollen upon flowers of the same variety. C. W. H. Heideman[2] made some very interesting observations on what he considers distinct forms of the flowers of the Americana plums, describing for this species all of the six possible variations of flowers enumerated by Darwin in his Different Forms of Flowers in Plants of the Same Species. Heideman thinks that other species of Prunus exhibit similar variations. Waugh[3] made the pollination of plums a subject of careful and extended study and found much variation in the pistils of plants of the same species, insufficient pollen in some plants, pollen impotent on the stigma of the same flower, and considerable difference in the time of maturity of pollen and stigma in some plums, especially the Americana plums. These variations, most important to the plum-grower, are of more or less use in identifying plums.

After the discussion of the characters of plums we may pass to a detailed description and discussion of the species of plums which now contribute or may contribute cultivated forms to the pomology of the country either for their fruits or as stocks upon which to grow other plums. The following conspectus shows as well as may be the relations of the species of plums to each other.

CONSPECTUS OF SPECIES OF PLUMS.

1. PRUNUS DOMESTICA Linnaeus

Beside the comparatively well-known groups of Domestica varieties, there are in Europe, with an occasional representative in America, especially in herbaria, numerous other groups either a part of Prunus domestica or possibly, in a few cases at least, hybrids between it and other species. European botanists place some of these in distinct species or sub-species; but few, however, even of the recent writers on the botany of the plum, agree at all closely as to the disposition of these edible and ornamental plums which may be doubtfully referred to Prunus domestica. With this disagreement between the best European authorities where these plums have long been known, where some of them have originated, and all may be found in orchards, botanic gardens and herbaria, it does not seem wise at this distance to attempt a discussion of such doubtful forms. It is certain, however, that Borkhausen’s Prunus italica and Prunus œconomica, as given in the synonymy, are but parts of Prunus domestica, the first including the Reine Claude plums and the latter the various prunes. So, too, a wild form named by Borkhausen, Prunus sylvestris, is probably a part of Prunus domestica.

Bechstein[4] gave specific names to a number of plums which Schneider[5] holds are all cultivated forms of Prunus domestica. These names are not infrequently found in botanical and pomological literature, to the great confusion of plum nomenclature. The following are Bechstein’s species:—Prunus exigua, Prunus rubella, Prunus lutea, Prunus oxycarpa, Prunus subrotunda and Prunus vinaria.

The plum in which the world is chiefly interested is the Old World Prunus domestica. The Domestica plums are not only the best known of the cultivated plums, having been cultivated longest and being most widely distributed, but they far surpass all other species, both in the quality of the product and in the characters which make a tree a desirable orchard plant. How much of this superiority is due to the greater efforts of man in domesticating the species cannot be said, for the natural history of this plum, whether wild or under cultivation, is but poorly known. It is not even certain that these plums constitute a distinct species, there being several hypotheses as to the origin of the Domestica varieties. Three of these suppositions must be considered.

Many botanists hold that what American pomologists call the species is an assemblage of several botanical divisions. The early botanists distributed these plums in botanical varieties of one species. Thus Linnaeus, in 1753, divided Prunus domestica into fourteen sub-species, and Seringe, in 1825, made eight divisions of the species. Both of these men include in this species, among others, plums which we now place in Prunus cerasifera, the Cherry plums, and Prunus insititia, the Damsons and Bullaces. Nearly all subsequent botanists who have not made two or more species of it have recognized from two to several sub-divisions of Prunus domestica. It is possible that what are called the Domestica plums should be distributed among several botanical divisions. But it is difficult to find any differential character sufficiently constant to distinguish more than one species for the several hundred varieties of these plums now under cultivation. Nor are there any cleavage lines sufficiently distinct to indicate that the edible varieties of the one species should be sub-grouped.

In coming to these conclusions the writer has studied about three hundred varieties of Domestica plums growing on the grounds of this Station and about half as many more growing in other parts of the country, the whole number representing all of the various species and sub-species which other workers have made. The differences which have been most used to classify the varieties of Domestica in several botanic divisions have to do chiefly with the fruit, as size, shape, color and flavor, characters so modified by cultivation and selection that they are artificial and transitory and of little value in botanical classification. Moreover, the botanical groups which have been founded on these characters are much more indistinct than ordinarily in botany because of the merging at many points of one group into another. This indistinctness is greatly increasing year by year through the intercrossing of varieties. When the characters of no value to man, and, therefore, little modified by cultivation, are considered, it is scarcely possible logically to place Domestica plums in more than one species or to further sub-divide the one species.

The botanists who have divided the Domestica plums into either greater or lesser botanical groups do not define their divisions with sufficient accuracy to make them clearly recognizable. Neither do they give the habitats of the wild progenitors with sufficient certainty to carry conviction that the groups were brought under cultivation from separate ancestors. Also, the several botanists who hold to the multiple species theory for the Domestica plums do not agree as to the limits of the different groups and give to them very different specific or variety names, showing that they have widely different ideas as a basis for their classification.

A second theory is that Prunus domestica is derived from Prunus spinosa and that Prunus insititia is an intermediate between the two.[6] This hypothesis is based upon the supposition that when Domestica plums run wild they revert to the Insititia or Spinosa form. It is not difficult to test this theory. A study of the origin of the several hundred Domestica and Insititia plums discussed in Chapters III and IV of The Plums of New York does not show for any one of them a tendency to reversion or evolution to other species; nor do the descriptions indicate that there are many, if any, transitional forms. During the two thousand years they have been cultivated in Europe the Old World plums have been constant to type. Domestica seedlings vary somewhat but they do not depart greatly from a well marked type. Such very few striking departures as there seem to be are more likely to have arisen through crossing with other species than through reversion or evolution. This Station has grown many pure seedlings or crosses of varieties of Domestica within the species and has had opportunity of examining many more from other parts of the State, and none of these show reversion to the other two Old World species. Nor, as we shall see, is there much in what is known of the history of these three species to lead to the belief that the Domestica, Insititia and Spinosa plums constitute but one wild species or have arisen from one.

It has been remarked that there are few, if any, transitional forms between the Domestica and other European plums. It is a significant fact that Prunus domestica can be hybridized with other species of plums only with comparative difficulty, species of plums as a rule hybridizing very freely. This is as true with the Insititia and Spinosa as of other plums, there being few recorded hybrids of either of these species with the one under discussion. Quite to the contrary the varieties of the several pomological groups of Domestica plums hybridize very freely. If all were of one species we should expect many hybrids between the Domestica, Insititia and Spinosa plums.

We are now left with the third hypothesis, which is, as we have indicated in a preceding paragraph, that the varieties of Domestica plums belong to one species; or if they have come from more than one species the wild forms have not been distinguished and must have grown under much more nearly similar conditions than is the case with Prunus domestica and any other species. Without knowledge of more than one wild form, and in view of the intercrossing of the varieties of these plums it seems best to consider all as parts of one species, leaving to the pomologist the division of the species into horticultural groups founded on the characters which make the fruit valuable for cultivation.

Assuming, then, that the plums known in pomology as Domestica plums belong to one species, the original habitat of the species may be sought. In spite of the great number of varieties of plums now grown in Europe and western Asia, and the importance of the fruit both in the green and dried state, the history of the plums cannot be traced with much certainty beyond two thousand years. Though stones, without doubt those of the Insititia or Damson and the Spinosa or Blackthorn plums, are found in the remains of the lake dwellings in central Europe[7] the pits of Domestica plums have not yet come to light. In the summer of 1909 the writer, in visiting historic Pompeii, became interested in the illustrations of fruits in the frescoes of the ancient buildings, but neither in the houses of the ruined city nor in the frescoes in the museums in Naples could he find plums, though several other fruits, as apples, pears, figs and grapes were many times illustrated. An examination of the remains of plants preserved in the museum at Naples taken from under the ashes and pumice covering Pompeii gave the same results. No stone-fruits were to be found, though if widely used these should have been on sale in the markets of Pompeii at the time of the destruction of the city, which occurred late in August,—the very time of the year at which the examination was made and at which time plums were everywhere for sale in Rome. This observation is but another indication that plums were not well-known before the beginning of Christianity, since Pompeii was destroyed in 79 A. D. In Greek literature the references to plums are few before the Christian era and these are more likely to some form of Insititia, as the Damsons, rather than to the Domesticas.

While the records are somewhat vague it is probable that the Domestica plums came from the region about the Caucasus Mountains and the Caspian Sea and especially the section east of these mountains and the sea. What seems to be the wild form of this species has been found by several botanists in this great region.[9] Here the Huns, Turks, Mongols and Tartars, flowing back and forth in tides of war-like migration, maintained in times of peace a crude agriculture probably long before the Greeks and Romans tilled the soil. The plum was one of their fruits and the dried prune a staple product. Here, still, to the east, west and north toward central Asia, plums are among the common fruits and prunes are common articles of trade. Even in the fertile oases of the great central Asian desert, plums are cultivated, but whether domesticated here or brought from elsewhere cannot be told. Koch,[10] speaking of prunes in particular, gives the following account (translated) of their Asiatic origin: