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 progressive or retrogressive character of the vegetation of mountain-tops, where the soil is constantly changing its position, demands a close study of the ground, so that the order of succession of the colonizing plants, whether initially cryptogams or not, may be elucidated. The open or closed nature of the formation requires a similar method in either case, and a survey of the surface in detail is essential. The special adaptations of the plants, owing to the particular conditions, such as wind, slope, soil, depth, sun, rainfall, snow-line, insolation, etc, are only to be determined by such methods.
The exact altitudes attained by each species, and the aspect and extent of the formation, association, or society, are other factors that are of especial interest. As a rule, the study of highland vegetation is open only to the favoured few who live in the vicinity of such areas. But it is suggested that an attempt be made to study a typical area, by visiting one of the mountainous areas in summer or autumn.
In the more generally accessible lowland areas the relation of the vegetation upon bare rocks to the more stable vegetation upon the deeper soils derived from them may be studied in the same way and with greater ease.
The flora of a wall, as it is artificial, requires rather different treatment; it need not be merely a floristic study or enumeration of species upon particular walls, but may also be studied from the wider ecological standpoint. The position of mural plants upon the wall, their relation to aspect, the extent of each society, the adaptations to dry conditions, are some of the points that should be considered.