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.
It is of advantage to plants that are exposed to dry-soil conditions to continue their activities from year to year. By this means, with a rest period in winter, they are enabled with the renewal of their vigour in spring to cope with the more disadvantageous conditions of the summer. The possibility of the germination, growth, and development of an annual is remote, as a rule, upon heath land. It is, of course, necessary that such hemi-parasites as Eyebright and Cow-wheat should come up from seed each year and attach themselves to the Grasses upon which they grow.
It is also of advantage to the dominant types of heath vegetation that they are shrubs and evergreen in many cases. This enables them to cope the more easily with the conditions, as they have not to renew the shoots each year.