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.
Prostrate trailing herbaceous perennials or undershrubs. Flowers usually racemose. Valves and placentas of the capsule 3. About thirty species are known. The name is derived from the sun, and a flower. Some species have dimorphic flowers, similar to the violet. They are only suitable for rock-work. Besides H. vulgare there are three other indigenous species : H. guttatum, an annual, flowers yellow with a red eye; H. canum and H. polifdlium, trailing shrubby species, the first with small yellow flowers, and the latter white,
1. II. Algarvense, syn. H. ocymoides. - This is an erect-growing species about 2 feet high, more like a Cistus, to which the commonly cultivated annual species, with white, lilac, purple, or crimson flowers.