The object of this work is threefold. In the first place it is intended as a general and descriptive account of typical areas of semi -natural vegetation in Great Britain. Secondly a study of the effect of biotic factors which produce this type of vegetation is involved, and special attention is given to a particular aspect of the biotic factor.i.e. the mechanical aspect. This latter influence has received little attention up to the present. Thirdly the economic applications of the study are investigated. It is very notable that almost all studies of vegetation in this country and certainly all vegetation problem: related to agriculture have been approached from the chemical or edaphic standpoint. The explanation lies in the fact that the earliest investigators were chemists, and their outlook has been acquired by the student, the investigator and the agriculturist. It is hoped to show in the following work that the solution to many ecological problems, especially those of semi -natural vegetation,is to be found by a consideration of some purely mechanical force ;and that the edaphic factor does not always exert the influence attributed to it. The results of the investigations have been exploited economically, in several directions. For example it is found that on the poorest grassland (from the graziers' point of view). that the footpaths traversing it, possess an entirely different flora of species with a high feeding value. A complete understanding of the factors producing this contrast enables the same changes to be produced in the field as a whole. Again in the sphere of weed cont, where mechanical agents are responsible for the presence or the propagation of certain weeds, a knowledge of these agents, and their mechanism, is shown to solve the problem of prevention and control. The influence of domesticated and wild animals upon vegetation is also shown to be due frequently to a mechanical rather than a chemical influence.