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.
Alpine plants, as has been seen, are largely confined to regions where there are accumulations of mountain-top detritus, or where, as in the case of chomophytes, corries or ravines occur; they are generally found above the limits of tree-growth at altitudes of 2000 ft. and upwards.
As a rule there is a decrease in temperature as one ascends, and this is the reason presumably for the retreat of the alpines to the mountains of this area; in the Ice Age they occurred at lower levels, owing to the lowering of the level of the snow-line, etc. The conditions are therefore much colder, and though the rainfall may be excessive, as at Satterthwaite, in the Lake District, the plants suffer from physiological drought, or in many cases transpiration is easy, so that the conditions are similar to those of dry soil.
The exposure of such elevated tracts to the wind also demands a rosette habit, or a grass habit, whilst other plants form mats or cushions, approximating to the habits of lichens, mosses, etc. The viviparous condition, a vegetative mode of reproduction, is frequent amongst such forms.
It is extraordinary that amongst the numerous rare specialized alpine forms growing at very high altitudes many lowland plants occur, and these must be regarded as descending species. The upright Meadow Crowfoot is found at 3980 ft., Marsh Marigold, Red Campion, Coltsfoot, and Hogweed at about 3500 ft., Bird's-foot Trefoil at 2800 ft., and Bog Violet ranges to 4000 ft.
In more lowland areas similar dry-soil conditions are encountered by plants growing upon sub-alpine hills, such as Rock Cress, Alpine Thale Cress, Rue-leaved Saxifrage, Orpine, Rock Rose, Wall Lettuce, etc.