This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
This differs from the last genus in having no tendril at the tip of the petiole, and is usually united with it; but for our purpose it is more convenient to keep it separate. The Latin name. Bitter Vetch is the common English appellation.
1. 0. vernus. - A valuable early flowering species. It grows about a foot or 18 inches high. Leaves composed of about three pairs of lanceolate acute leaflets. Flowers reddish purple, changing to pale blue. A native of Europe, flowering in April and May.
2. 0. Iuteus. - A handsome plant of about the same stature as the preceding. Leaflets about 8, elliptical, rather acute, glaucous beneath. Flowers yellow. Siberia.
3. 0. niger. - An indigenous mountain species, 1 to 2 feet high. Leaves of 3 to 6 pairs of oblong-lanceolate acute leaflets. Peduncles equalling or exceeding the leaves, 2- to 8-flowered; flowers livid purple, fading to blue.
4. 0. dibus. - This is a distinct species with about 6 narrow linear-lanceolate acute leaflets. This species is very variable in the colour of its flowers, ranging from white and cream tinged with rose to purple and white and yellow. It includes 0. varius, versicolor, and canescens, A native of Austria, flowering in Spring.
Besides the above there are others equally effective, as 0. variegatus, rose crimson and blue striped; 0. coceineus, scarlet; Jordani, blue; and tuberosus, red fading to blue.