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.
As a hill is ascended the temperature is lowered by 1° F. for each 300 ft. Hence the montane species of flowering plants require a longer period of sunny weather before the thermal constants demanded by each species are respectively reached. The persistence of moist conditions or rainfall may also retard flowering to some extent, as also the effect of exposure and the wind. All these circumstances contribute towards the generally late-flowering seasons of the plants of the hillside, a fact easily demonstrated by a comparison between a lowland and an upland meadow; and the well-known purple tint of the moors late in the summer or in autumn is a familiar illustration of this fact on a wide scale.
The chalk hills of Boxhill and elsewhere furnish an example of the early flowering of a hillside plant, the Box. This may be due to the close habit of growth of the Box, the sheltered character of the habitat, and its southern origin and distribution. The Hairy Violet, another chalk species, like all the Violets is early, flowering also in June. In May the Rock Rose, Kidney Vetch, Clary, and Sheep's Sorrel are in flower, all but the last, it should be remarked, being chalk plants.
June, however, is the principal month for the hillside plants to flower. Then we may find the following: Touch-me-not, Rest Harrow, Silky Mountain Vetch, Sainfoin, Drop-wort, Oak-leaved Mountain Avens, Wild Thyme, Musk Orchid, Fragrant Orchid, Sheep's Fescue. Many of these, again, are limestone or chalk plants. In July, Musk Mallow, Hare's Foot Trefoil, Salad Burnet, Field Scabious, Ploughman's Spikenard, and Cotton Thistle flower, and the Gentians do not bloom till August. On the whole it is thus to be noticed that hillside plants flower late.
There are several reasons why the duration of hillside plants should be more or less uniform. In the first place, the late flowering of the plants of upland regions, which has already been pointed out, necessitates this; for it is impossible for plants that are subjected to conditions prevalent in highland regions to mature in the same manner as those that grow in the lowlands. Hence it is that with a few exceptions the upland plants are perennial. They form frequently wide associations, which are generally made up of perennial plants, such as Grasses, Sedges, Heaths, etc.
At higher elevations there are not many plants of annual or biennial duration; for the ground is not open or broken, but taken up more or less continuously by plants of vigorous dominant growth. Cotton Thistle, the common Gentians, Touch-me-not, and Hare's Foot Trefoil are annual or biennial. It is thus clear that the hillside plants have adapted themselves to perennial conditions as a result of the factors that govern the upland floral regions.
The uplands consist largely of pastures, and there is as a consequence a parallel between the conditions for insect life on the hills and the lowlands, where pastures are also predominant. The hills are, in fact, as much alive with the busy hum of the insects in spring and summer as the lowland meadows. This may be seen, indeed, in the existence of a large number of insect pests that live as larvae; upon the hillside plants.
The openness of the hillside contributes to the prevalence of insect life, for insects can wander at will unimpeded by any barriers as in woodlands, or where the type of habitat is restricted as in the case of a cornfield, a bog, heath, etc. The hillside plants as a whole have brilliantly-coloured flowers, conspicuous and attractive, as Rock Rose, Marsh Mallow, Kidney Vetch, Sainfoin, Field Scabious, Clary, Musk Orchid, Fragrant Orchid, etc.
Salad Burnet, Sheep's Sorrel, Sheep's Fescue are pollinated by the agency of the wind.