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.
Heath vegetation may be either open or closed in character, woodland in the latter case alternating with more open conditions. But everywhere it is never so protected or sheltered as in a true woodland.
In the first case the heath is exposed to the full glare of the sun. The flowers that bloom in the open association are not usually large and conspicuous, but they are frequently sweet-scented, e.g. Furze and Broom, and Ling, Heather, Hawkweed, Sheep's-bit Scabious, and Harebell are also attractive to insects. The masses of Ling and Heath, Broom or Furze together form a conspicuous feature; hence by association they attain the same end that individually conspicuous flowers do in the matter of attracting insects. On a small scale the Blue Milk Wort, Starry White Grassy Stitch-wort, Pretty St. John's Wort, Tormentil, Heath Bedstraw, Eyebright, Red Rattle, Cow-wheat, are also attractive.
In the case of the tree types and the Spike Rushes, Sedges, and Grasses, as a rule the flowers are proterogynous, the stigma is feathery, and the plants are pollinated by aid of the wind. The flowers of the Creeping Willow are also resorted to by bees. Thus the vegetation of the heath is adapted equally to cross-pollination by insects and to wind pollination, its open character being advantageous in both respects.