This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
Habitats in general may be wet or dry, rocky or not, upland or lowland. The wettest habitats (especially E. Anglia) are afforded by the Alder-Willow associations, where the scrub and ground flora is made up of such plants as Guelder Rose, Currant, Gooseberry, Meadowsweet, Yellow Flag, Reed, etc, Bitter-cress, Kingcup, Figwort, Great Hairy Willow-herb, Tussock Grass, etc. The tree types are scattered, and the ground itself is open, allowing such large herbaceous plants to thrive.
On clays and loams, sandy and siliceous soils, the trees are Oak, pedunculate and sessile, Birch, with other trees. The scrub and ground flora are very variable. The woodland may be close or open. On clays and loams the ground flora is largely gregarious, e.g. Bluebell, Bracken. On sandy soils more often there is a good deal of bare rock surface with deep soil elsewhere, giving a variety of habitats. The same applies to a Birch wood. Here also there are wide associations of grasses, such as Heath Hair Grass, Matweed, etc, and the heaths also form wide gregarious associations. These habitats are largely upland, whilst the oak-woods are mainly lowland, the sessile Oak not growing above 1500 ft. as a rule. The pedunculate Oak has wet-soil conditions, the sessile being adapted to dry-soil conditions.
The woodlands on calcareous soils (Carboniferous Limestone, Chalk, Oolite) afford in the case of Ash woods varied habitats or types of association, the plants being often gregarious, as in the case of Dog's Mercury and Archangel. They rise to some altitude. On marls the Ash-Oak woods are variable in the types of association. In the case of Beech woods the habitat is upland very largely, and dry, there being a scanty scrub and ground flora.
The wet or dry character of the woodland determines largely the nature of the habit. Thus in Alder-Willow woodland associations the plants, such as Rushes, Grasses, and Sedges, with the grass habit are largely tufted or caespitose. Even the rosette types are frequently tufted, as in the case of Marsh Marigold and Bitter-cress. The procumbent or trailing habit is also characteristic. These habits are transitional to the submerged and floating habits of purely aquatic plants, which are intimately associated with fen formations.
In the normal dry woodlands the tree habit is the dominant one. The scrub is analogous to that of the tree habit, but is always influenced by the tree zone. The stems and branches are less strong, and thick, the leaves are small, and often several times pinnate. Spines are more numerous, and the flowers are more suited in most cases to pollination by insects.
The ground flora is variable in habit. There is the climbing habit of the Ivy or the Honeysuckle, etc, adapted to reliance upon the support of trees or scrub. The bulbous or tuberous habit is especially typical, e.g. Bluebell, Orchids. A large number of plants are prostrate or procumbent, or provided with creeping underground stems, as Strawberry, Wood Anemone.