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.
Limestone, in the last section, refers to the Palaeozoic limestones, which are developed in the moister west and north of England. The chalk and oolites, also limestones, which are not so compact or altered, are more porous, and, being confined to the southern and eastern parts of the country, are drier, and many of the chalk plants are therefore xerophiles.
The typical woodland is Beech, but Ash woods also occur. They occur on the slopes or hangers (a name familar to readers of White's Selborne), growing often on the rock itself, or where the soil is merely a shallow layer of mild humus. With the Beech grow Ash and White Beam, and, locally, Box (as on Boxhill) and Cherry. The Yew is also a characteristic tree, native here. In the Beech wood there is little scrub, but on calcareous pastures or the margins of the chalk, White Beam, Cornel, Buckthorn, Wayfaring Tree, Spindle Tree, etc, are found.
The scanty ground flora consists of Wood and other Violets, Wild Strawberry, Enchanter's Nightshade, Sanicle, many Orchids, Bird's-foot, Belladonna, Butcher's Broom.
In addition to Beech, Ash, and Yew woods upon the chalk, there is a scrub association and a chalk pasture. In the last case the soil is often not more than 1 in. in depth, but an extensive flora occurs upon it, with a turf of Sheep's Fescue, or, in other cases, of Bromus erectus, Trisetum flavescens, Brachypodium sylvaticum. Occasionally the soil disappears, and here the plants are subjected to intensely dry conditions in a hot summer.
These gradations of soil thickness, again, show that between rocks pure and simple and rock soils there is very little difference, especially in this case. But where a non-calcareous soil is formed above the chalk, as frequently happens, this is not the case.