250 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



and restricted. Consequently some regions have a characteristic 

 weed-flora of their own, to which many of the component species 

 are largely limited. Thus whereas many weeds have nowadays the 

 type of distribution exhibited by some crops, that is, practically 

 world-wide within climatically limited bounds, others are restricted 

 by special conditions to particular areas for which they are suitable. 

 In the former case peripheral portions of the distribution are often 

 very temporary besides being artificial, Man having extended the 

 area far beyond its natural bounds. Moreover, as we might expect 

 from examination of the ranges of wild plants, weeds in areas outside 

 those of their climatic optimum tend to be restricted to particular 

 habitats where such factors as soil type or microclimate compensate 

 for unfavourable climatic conditions. Here even vigorous weeds 

 may be only sporadic in appearance, easy to control, and liable to 

 disappear quickly in the absence of human interference. Neverthe- 

 less, many of the weeds which follow particular crops are almost as 

 widespread as the crops they accompany. 



Modern plant pathology, the scientific study of plant diseases, is 

 a large and important subject in its own right — as may be gathered 

 for example by perusal of such works as those of Butler & Jones, 

 Walker, and Boyce cited at the end of this chapter. Only a few 

 selected distributional and allied items can be touched upon here, 

 for almost any abnormal state of plants, such as may depress the 

 yield of a crop, is liable to be considered as a disease. 



Plant diseases may conveniently be classified into three primary 

 groups : (i) non-parasitic, incited primarily by such physical or 

 chemical factors as low or high temperatures, unfavourable oxygen 

 or soil-moisture relations, atmospheric impurities, lightning, and 

 mineral and other excesses or deficiencies ; (2) parasitic, incited by 

 Bacteria, various groups of Fungi and their allies, Angiosperms, 

 and animals such as insects and Nematode worms ; and (3) virus 

 diseases. Examples of most types can be lethal — sometimes to all 

 the plants belonging to a particular species throughout a tract of 

 country. They are therefore of great importance to plant distribu- 

 tion ; for they are apt in some cases to attack any part of the area 

 of a particular species or even wider taxon, and consequently to be 

 of the utmost significance to mankind whose crops they so frequently 

 affect adversely or ruin or even basically destroy. 



Whereas very numerous and often serious diseases are known of 

 wild plants throughout the world, it is chiefly among cultivated ones 

 that the worst ravages are caused or at all events noted. Here 



