There are two forms with brown bark which have been at various times regarded as separate species, or as mere varieties, but which Sir J. D. Hooker ranks as sub-species, marking a stage in which varietal characters have become permanent, but not sufficiently strong to hide their connection with the parent form. These are:--
I. The Bullace (_P. insititia_), with larger and broader leaves, underside downy in the adult condition; branches straight, only a few with spines; the petals broader; the fruit more drooping, black or yellow, larger, and less rough to the taste.
II. Wild Plum (_P. domestica_). Branches straight without spines. Fruit larger, black. Leaves downy on the ribs of the underside. The plums of the fruiterer and the “prunes” of the grocer are cultivated forms of this species.
They all flower in March and April. The name is the old Latin appellation for the fruit.
The Wild Hop may not unfrequently be seen in the copse and hedgerow, especially in the South of England. It has a thick branching perennial rootstock--in the cultivated plant called a “set”--from which are produced several long, thin, but tough twining stems that turn with the sun, and tightly clasp the nearest small tree or shrub. It has no tendrils like the vine, but climbs like the convolvulus by simply twining with the sun as it grows. Its lobed and coarsely toothed leaves are very similar to those of the grape-vine, but very rough. The leaves are in pairs, and at the base of the leaf-stalk is a pair of long curved stipules. The Hop is what botanists term a _diœcious_ plant, because staminate flowers only are produced by one individual, and pistillate only by another, making cross-fertilization imperative. It is not the insects, however, that effect this crossing in the Hop, but the wind. The flowers are all small; the staminate produced from the axils of the leaves in long drooping panicles. They have no petals, but there are five sepals and five anthers attached to their bases. Each pistillate flower has a membranous sepal, an ovary, and two long tapering purple stigmas. Two of these pistillate flowers are produced in the axil of a green, broad, concave bract or scale. A number of these twin-flowered bracts are united into a dense spike, and after fertilization this develops into a large cone-like head of yellow scales with resinous glands at their base, which yield a resinous substance called _lupuline_. The true fruit is a little nut, which is enclosed in the sepal under the bracts. It flowers in July and August. It is the only British species. Beside their extensive use in brewing, the flowers are frequently used to stuff pillows, their narcotic odour inducing sleep.