_Physiological Botany_ deals with the life processes of plants, and the way in which they feed and grow and move. It has a very important bearing on the distribution and grouping of plants, since this is largely governed by their food-supply and by the need of surroundings which allow them to carry on their life processes with success. It will be seen that there are many lines of enquiry open to the student of botany. In the following pages no more can be attempted than the preliminary study of some of the more familiar phenomena of plant life as it presents itself to the holiday-maker on the hills and woods and shores of our own land. CHAPTER II PLANT ASSOCIATIONS “It is perhaps also proper to take into account the situation in which each plant naturally grows or does not grow. For this is an important distinction, and specially characteristic of plants, because they are united to the ground and not free from it like animals.”--THEOPHRASTUS: _Enquiry into Plants_, I. iv. Before setting about discussing the various types of vegetation which our own country presents, it will be well to have a general idea of the extent to which the main types are developed, and of the amount to which agriculture has interfered with the native flora. We have seen that the natural vegetation of the greater part of the British Isles is woodland: yet so profoundly has human industry altered the face of the country that woodland, natural or planted, occupies only about one-twentieth of the surface of England, rather less of Scotland and Wales, and about one-seventieth of Ireland. Much of the former woodland is now represented by “arable land,” which covers over one-third of England, and about half that proportion of the other parts of the British Isles. Permanent grassland, partly natural, partly replacing ancient woodland, bulks large in England and Wales, occupying about two-fifths of the whole country; in Scotland and Ireland the proportion is much less, but in those countries a large area is under moor, heath, or natural grass, over which wander great herds of sheep and cattle. A. G. Tansley[3] thus contrasts (in percentages) the area of cultivated land (on which natural vegetation has been to all intents destroyed), with the area on which natural or semi-natural conditions still prevail: England. Wales. Scotland. Ireland. Cultivated land 75 59 25 ? 20-30 Land under natural or semi-natural vegetation 15-20 40 70-75 ? 70-80