This section is from the book "Wayside And Woodland Blossoms", by Edward Step. Also available from Amazon: Wayside And Woodland Blossoms: A Guide To British Wild-Flowers.
It seems quite natural to use the two common names of this beautiful shrub at different times. In the spring, before a leaf has unrolled upon the spine-tipped spurs of its soot-coloured branches, we call it the Blackthorn, for by contrast with its pure white stars its thorns are black indeed. In the autumn, when we search the common, the copse-side and the thick hedgerow for ripe bramble fruit, we only know it as the Sloe. Then the plant is again in full beauty with its groups of round plums, each finely coated with the purple bloom that is ruined by a touch. Like the Whitethorn (page 17) and the Bramble (plate 30), the Blackthorn is a rose, with the floral organs in fives. The fruit is botanically a drupe: it is the result of a swelling up of the ovary, the outer walls of which become succulent and pulpy, the inner hardened into the "stone" inclosing the "kernel" or seed. The leaves are small, elliptical, finely toothed, and in a young state the underside is downy, but in the adult condition smooth. All the branches are spiny.
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.
Prunus communis. - Rosaceae, -