Chapter VIII 



MODIFICATION AND DISTRIBUTIONS 

 OF CROPS (AND WEEDS) 



It is now time to deal with the so-called ' artificial ' changes 

 wrought by Man, whether intentionally or accidentally, in the 

 distribution of plants. In this connection Man seems during recent 

 centuries to have been the most potent factor in the world ; and 

 as his activity increases and more and more barriers are broken 

 down by transport, his effectiveness as a distributor grows ever 

 greater. This transportation is quite apart from the changes Man 

 brings about incidentally in the course of his ever-extending activities 

 of husbandry or desecration. 



From what was said in the last chapter it should be clear that, 

 whereas plants have their own distribution patterns, and particular 

 taxa have particular areas which they are capable of occupying, it 

 is rarely if ever that a vascular plant taxon will occupy anything 

 like the whole of the geographical area or areas where the climate 

 is suitable for it. Usually, numerous unsuitable habitats will inter- 

 vene, and even then there are commonly left areas of suitable 

 habitat which the plant in question has failed to reach, or in which, 

 if it has arrived, it has failed to establish itself and survive. In 

 other words, the present areas occupied by particular plants tend 

 to fall far short of the maximum which they are capable of occupy- 

 ing : artificial introduction of a plant outside its present natural 

 area will frequently demonstrate its ability to grow in a wider range 

 of situations both geographically and ecologically. Thus, besides 

 its own natural area of distribution, each species has, at least in 

 most instances, a wider potential area which, if we include places 

 where it can be grown in cultivation or otherwise in the virtual 

 absence of competition, is often very much more extensive. This 

 principle is of great significance in connection with the introduction 

 and production of crops on which Man largely depends. 



The dispersal of plants by Man was considered in a special section 

 of Chapter IV. Here we must deal with the results of such transport 

 so far as the all-important crops of field and forest and the recognized 



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