There has been much discussion as to whether Prunus nigra should be given specific rank or be united with Prunus americana, either as a part of that species or as a botanical variety of it. Until the revival by Sargent in 1892 of the name given the group by Aiton in 1789, the botanists of the latter half of the Nineteenth Century had for most part described the two groups under Prunus americana. Since Sargent’s re-establishment of the species, botanists have very generally regarded it as worthy of the rank. Bailey and Waugh, the leading horticultural authorities on plums, however, consider Nigra as but a botanical variety of Americana. The taxonomic characters of Prunus nigra seem to the writers of The Plums of New York to be as distinct as those of several other of the native species of Prunus, and since the species now is generally recognized by botanists, we have considered it in this work as distinct from Prunus americana.
The two species may usually be distinguished by the following differences: (1) The general aspect of the trees is very different. The tree of Americana is larger, the top is more spreading, and its branches longer, with more twigs, more slender and more pendulous. The bark on the trunk is lighter-colored and much more shaggy than in Nigra. (2) The wood of Nigra is tougher and the trees do not break as readily as those of Americana. The wood is also lighter in color. (3) The leaves of Nigra are larger, broader and the serrations are not so deeply incised nor so often double. Very distinct and very constant are the glands to be found on the teeth of the serrations on the young leaves of Nigra. These glands disappear as the leaves grow older, leaving a calloused point which makes the serrations of Nigra rounded, while those of Americana are acute, this being one of the most constant differences. (4) The flowers of Nigra appear several days earlier, are larger and are more pink than those of Americana. (5) The calyx-lobes of Nigra are glandular and the leaf-stalks are biglandular, characters usually not found in Americana. The calyx in all its parts is glabrous or at least far less pubescent than in Americana and if present the hairs are short and stiff, whereas in Americana the pubescence is soft. (6) The fruit of Nigra ripens earlier and is darker in color with less bloom and is more oblong than that of Americana. The skin of the plum is thinner and is not so objectionable either cooked or eaten out of hand. (7) The stone of Nigra is usually larger, flatter and more strongly crested. The characters of the two species vary much in different individuals and there are many intermediate forms but the differences seem as constant as between other species of this variable genus.
The Nigra plums are important horticulturally because they can be grown in somewhat colder regions than the Americanas. They not only endure more cold than the last named group, but their tough wood enables them to stand better the weight of snows and the stress of winds. Their earliness, too, prolongs the season for this type of fruit and in regions where the season is short they may be grown with more certainty than other groups. In habits and characters other than those named they are so like the Americana as to need no further discussion. About forty varieties of this species are under cultivation.
13. PRUNUS ALLEGHANIENSIS Porter
1. Porter Bot. Gaz. 2:85. 1877. 2. Ibid. Gar. and For. 3:428, fig. 53. 1890. 3. Sargent Sil. N. Am. 4:27, Pl. 153. 1892. 4. Bailey Ev. Nat. Fr. 225. 1898.
Tree low, slender, straggling, fifteen to eighteen feet in height, or a low shrub; trunk-diameter from five to eight inches; bark dark brown, surface fissured and scaly; branches numerous, upright, rigid, seldom spiny; branchlets pubescent, becoming glabrous and red, turning to dark brown; lenticels many, small, white.
Leaves ovate-oblong or lanceolate, sometimes obovate, apex acute or acuminate, base rounded, margin sharply serrate, teeth fine and tipped with glands, in texture thick and firm; upper surface dark green and glabrous; lower surface light green, glabrous except on the veins and midrib; petioles short, slender, pubescent; glands two, large, at the base of the blade.
Flowers white, fading to pink, one-half inch across, appearing with the leaves; borne in two to five-flowered umbels; pedicels slender, finely tomentose, from one-fourth to one-half inch in length; calyx-tube narrowly obconic; calyx-lobes entire, pubescent on the outer, tomentose on the inner surface; petals rounded but narrowing into claws at the base; filaments and ovary glabrous; anthers often reddish; style slender with a funnel-shaped apex.
Fruit matures in summer or early autumn; from one-quarter to three-quarters inch in diameter, sub-globose or ovoid, dark blue or purple with heavy bloom; skin thick and tough; flesh yellow, juicy, acid and somewhat astringent; stone clinging to the flesh, turgid, acute at the ends, thin-walled, ridged on the ventral and grooved on the dorsal suture.
In leaf, flower and tree Prunus alleghaniensis resembles Prunus americana. The species has long been known to be distinct, however, having been first distinguished by J. R. Lowrie of Warriorsmark, Pennsylvania, in 1859,[122] and was published as such in 1877, when T. C. Porter of Lafayette College described it as Prunus alleghaniensis. It differs from Prunus americana chiefly in the smaller size of the plant, smaller leaves and flowers, in color of flowers which fade to pink in this species, and in fruit-characters. The fruit matures earlier, is much smaller in size, is more globose, and is a dark purple or blue with very heavy bloom. The skin is thick and tough and while the texture of the flesh is as good as that of the wild Americanas the flavor is much more astringent. The stone is more swollen. The plant is commonly but a shrub, usually found along fence rows and the borders of woodlands, but intermingled among old thickets of this kind there are often a few small trees. It is a hardy species, very productive, and seemingly but little attacked by either insects or fungi, being especially exempt from black-knot.[123] In the wild state it produces great numbers of suckers which seem to spring very readily from a bruise or an exposed root.