Tree reaching a height of 30 or 40 feet, vigorous, open-headed, round-topped; trunk attaining a foot or more in diameter; bark thick, ashy-gray with a tinge of red, nearly smooth or roughened with transverse lines; branches upright or spreading, straight, stout and rigid, usually spineless; branchlets usually pubescent, light red the first year, becoming much darker or drab; lenticels small, raised, conspicuous, orange.
Winter-buds large, conical, pointed, pubescent, free or appressed; leaves large, ovate or obovate, elliptical or oblong-elliptical, thick and firm in texture; upper surface dull green, rugose, glabrous or nearly so, the lower one paler with little or much tomentum, much reticulated; margins coarsely and irregularly crenate or serrate, often doubly so, teeth usually glandular; petioles a half-inch or more in length, stoutish, pubescent, tinged with red; glands usually two, often lacking, sometimes several, globose, greenish-yellow; stipules very small, less than a half-inch, lanceolate, narrow, serrate, early caducous.
Flowers appearing after or sometimes with the leaves, showy, an inch or more across, greenish-white to creamy-white; borne on lateral spurs or sometimes from lateral buds on one-year-old wood, 1 or 2 from a bud in a more or less fascicled umbel; pedicels a half-inch or more in length, stout, green; calyx-tube campanulate, glabrous or pubescent, green; calyx-lobes broadly oblong, obtuse, pubescent on both surfaces, glandular-serrate, usually reflexed; petals white or creamy in the bud, oval to obovate, crenate, notched or entire, claw short and broad; stamens about 30, equal to or shorter than the petals; anthers yellow, sometimes tinged with red; pistils about as long as the stamens, glabrous or pubescent.
Fruit of various shapes, mostly globular or sulcate, often necked, blue, red or yellow; stem a half-inch or more long, stout, pubescent; cavity shallow and narrow; apex variable, usually rounded; suture prominent or sometimes but a line or indistinct; skin variable; dots small, numerous, inconspicuous; flesh yellowish, firm, meaty, sweet or acid and of many flavors; stone free or clinging, large, oval, flattened, blunt, pointed or necked, slightly roughened or pitted; walls thick; one suture ridged—the other grooved.
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.