lO] ENVIRONMENTAL FACTORS 3II 



Moreover, in such arid lands as Mesopotamia, whence western 

 civiHzation apparently sprang, successful cultivation also involves 

 the artificial supply of water by irrigation. Conversely in many 

 excessively humid or waterlogged areas, drainage is necessary before 

 most crops can be sown. 



As a particular crop will remove more and more of the same (often 

 necessary) substances from the soil, and perhaps add more and more 

 of the same undesirable ones, it is common either to leave land 

 fallow {i.e. cropless) for one year out of every two to four, or else 

 to practise a ' rotation ' of different crops grown successively on an 

 area. In such cases more or different weeds will be fostered, with 

 obvious plant-geographical implications. Indeed the practices of 

 and incidental to cultivation, such as removing natural or semi- 

 natural vegetation, establishment (however temporarily) of artificial 

 vegetation in the form of crops, the introduction of weeds and diseases 

 whether they are controlled or not, and the opening up of fresh 

 areas for plant colonization and succession, have a continuous and 

 very widespread effect on the distribution and luxuriance of flora 

 and vegetation in the world, and, accordingly, on local landscape 

 and amenities. With Man's predominant position in modern times, 

 his biotic or, more precisely, ' anthropic ' influence has become 

 widely overwhelming. 



Further CoNsmERATiON 



Three small introductory books, treating inter alia the factors of the 

 environment, are : 



Sir A. G. Tansley. Introduction to Plant Ecology (Allen & Unwin, 



London, pp. 1-260, 1947). 

 William Leach. Plant Ecology, fourth edition (Methuen, London, 



pp. vii + 106, 1956). 

 H. Drabble. Plant Ecology (Arnold, London, pp. 1-142, 1937). 



Further useful treatments, which are mostly more detailed but by no 

 means uniform in their groupings and terminology, include : 



A. G. Tansley & T. F. Chipp. Aims and Methods in the Study of 

 Vegetation (Crown Agents for the Colonies, London, pp. xvi -[- 383, 

 1926). 



J. Braun-Blanquet. Plant Sociology (McGraw-Hill, New York & 

 London, translated, etc., pp. xviii + 439, 1932). 



J. E. Weaver & F. E. Clements. Plant Ecology, second edition (McGraw- 

 Hill, New York & London, pp. xxii + 601, 1938). 



