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.
Walls are the artificial counterparts of rock habitats, and are dispersed uniformly over the country in the lowlands, as well as at higher altitudes. In the neighbourhood of villages and towns they are composed of a variety of materials. Where natural exposures of hard rocks do not occur, walls are largely composed of brick. The upper surface forms a habitat for a number of plants, where some detritus has accumulated, and here one may find Rue-leaved Saxifrage; but as it is a lime-loving plant to a great extent, it is more frequent in the interstices of the mortar, where Ivy-leaved Toadflax also grows.
There were formerly a large number of mud walls in differents parts of the country, a survival from the days when houses were similarly constructed of mud; barns and outhouses are frequently built of mud to-day, having in many cases a top or roof of thatch. This latter is a favourite habitat for Vernal Grass, Rue-leaved Saxifrage, Thale Cress, Biting Stonecrop, Houseleek, Field Speedwell, and Flat-stalked Poa.
Whilst walls vary in composition, the habitat is resorted to by the plants that occur upon it, not so much because of such differences, but because it is uniformly a dry habitat. Upon old walls, especially in less-frequented districts, a large number of plants may be found. Sometimes Furze establishes itself, whilst in sheltered, moist spots Navelwort may grow in the crevices, Shining Crane's Bill, Wall Rue, and many ferns.
Artificial Character of Walls. etc - The artificial character of walls requires emphasis in the recording of the rarer plants that are found upon them. The Wallflower is a plant which is seldom found anywhere but on walls outside gardens, and was certainly introduced in comparatively recent times. But its persistence on certain places upon old ruins, etc., is an interesting feature biologically. The part played by walls in the retention of seeds or spores is of importance. The direction of the prevailing winds and the orientation of the walls are points that should be studied in this connection.
The influence of walls, and of boulders which are dispersed by glacial action up and down the country north of the Thames, as well as of the numerous blocks of stone used to mark the boundaries of roads, is responsible in many areas for the sole persistence of some interesting plants. There are some doubtfully native British plants, as the Oxford Ragwort, which in this country are to be found as a rule only upon walls.