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 heath, except where dominated by trees, is, as a rule, a very open association. The wide stretches of Ling, Heath, or Whortleberry are frequently unvaried by a single bush or tree over wide areas. The same may be said relatively of a Furze common, but it is often studded here and there with a scrub of taller shrubs or trees, as Hawthorn, etc. The grass heath also is of a very uniform open type.
This openness of the heath exposes it to severe conditions, which cause the ericetal plants to be modified accordingly. In the first place, the absence of shade causes the plants to adapt themselves to dry conditions. The leaves are leathery, rigid, narrow or linear, the margins inrolled, and the plants are often hairy. The exposure of heath plants to the wind is also a characteristic feature, and they have a typical habit correlated with this factor.
The open heath again lends itself to the successful pollination of the chief ericaceous types by insects. In the case of grass heath, on the other hand, the dominant Grasses are enabled to disperse their pollen, and later their seed, by the aid of the wind.