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.
Herbs or evergreen trailing shrubs. Leaves opposite, entire, glabrous and glossy. Flowers solitary, axillary, blue, white, or purple. Calyx 5-lobed, lobes glandular inside at the base. Corolla salver-shaped, the tube hairy within; lobes oblique. Stamens 5; anthers bearded. Disk biglandular. Carpels 2, many-seeded; seeds neither plumose nor winged. There are about 10 species, occurring in Europe, Asia, and Africa. The genus derives its name from vincere, to bind, the use made of the flexible branches.
1. V. major. Larger Periwinkle. - A trailing shrub with erect flowering-branches. Leaves ovate-cordate, ciliate. Flowers large and showy, bright blue, appearing in the Spring; calyx-lobes ciliate. There are several varieties, but the one called elegantissima, with beautifully variegated foliage, is the best. This species is a native of Europe and North Africa, and is occasionally found as a straggler from cultivation in Britain.
2. V. minor. Lesser Periwinkle. - This is smaller in all its parts than the preceding. Leaves ovate-lanceolate or elliptical, not ciliated on the margins. It blooms about the same time. There are blue; reddish purple, and white single- and double-flowered varieties, and others with gold or silver variegated foliage. This species is frequently met with growing wild in England, though not usually considered as indigenous. It is confined to Europe.
3. V. herbacea. - As the name implies this is of more herbaceous habit and less vigorous growth. The foliage, too, is less ample, and ovale or narrowly lanceolate, and rough on the margins. Flowers more abundant, with narrower corolla-lobes. A native of Hungary, flowering in Spring.