conditions of soil and moisture, have nevertheless a quite restricted geographical range. Although our own country, favoured by conditions thoroughly suitable to plant life--a sufficiently high temperature and an abundance of moisture and light--is characterized by a continuous plant mantle--or _closed vegetation_, as the botanists say--nevertheless what has been said of desert and semi-desert conditions applies to many limited areas in the British Isles, where the vegetation takes on the peculiar characters of true desert plants. Low water-content and great exposure produce such conditions on shingle beaches and sand dunes; and, as we shall see later, the vegetation of sea-rocks, salt-marshes, and peat-bogs is in many respects analogous to desert vegetation. Except near the Poles, wherever the precipitation of moisture rises above an amount which varies according to other conditions prevailing, a closed vegetation occupies the ground when the agricultural and other operations of man do not hold it in check. But as much of this favourable region is utilized by the human race for the production of plants used for food or for industry, it often happens, as in our own country, that the natural plant communities are to a great extent destroyed, and can be studied only on land left undisturbed because unsuitable for cultivation--on heaths and moors, in swamps and lakes, on sea-sands, chalk downs, and so on; and even in most of these places intensive grazing of domesticated animals and other causes connected with human activities alter and control plant life to a greater or less extent, rendering it necessary for us to walk warily in our study of it.