This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
The duration of plants that are found upon rocks and walls is regulated by various causes. The arctic-alpine vegetation is exposed to such adverse conditions that it is normally perennial. But a few annuals are found even amongst this group of plants, or of those that range as high, e.g. Vernal Whitlow Grass and Rue-leaved Saxifrage. Sandwort Spurrey may be either annual or biennial, and Houseleek is perennial or biennial. The Rampion is also a biennial. Grasses are usually perennial, but some are annual, e.g. Silky Wind Grass, Silvery Hair Grass, Sand Fescue.
The remainder of the plants here considered are perennial. This corresponds largely with an increased stature or size, or with a diurnal habit or late flowering period.