Chapter VII 



ARTIFICIAL FACTORS IN THE GEOGRAPHICAL 

 DISTRIBUTION OF PLANTS 



In examining the factors responsible for plant distribution, it is 

 necessary, first of all, to establish whether those aspects of their 

 distribution not explicable on the basis of present-day physiographic 

 conditions — isolated fragments of areas, relic and endemic species and 

 centers, various combinations of floras, etc. — are to be explained as 

 efifects of that most potent of modem factors, the activity of man, or 

 of more ancient, historical factors. If the former be the case, we have 

 no need to delve into complicated paleogeographical and paleoclima- 

 tological problems; if the latter, we should concentrate our attention 

 primarily on such problems and on other natural factors. 



When changes in the distribution of plants are induced, acciden- 

 tally or intentionally, by the action of man, we say that they are caused 

 by "artificial factors". Instances of man's influence on vegetation are 

 so numerous and well known to every one that there is no need of re- 

 counting them in detail. We shall here consider only general problems 

 connected with man's influence on plant distribution, with the aim of 

 determining the significance of man's activity as an explanation of 

 those phenomena in the geographical distribution of plants which are 

 of particular interest for our branch of science. 



Man's activities not only have an indirect effect on plant distri- 

 bution, e.g., by the destruction of natural plant communities, but man 

 acts as a direct agent in the transfer of plants from one locality to an- 

 other. Moreover, even as a direct agent, we may draw a distinction 

 between man's intentional change of habitat of a plant, often by 

 transfer over very great distances, and his involuntary aid to a plant's 

 change of habitat. In the former case we have in mind plants intro- 

 duced by man into cultivation — cultivated plants and their derivatives; 

 in the latter — weeds and other wild plants that have accompanied man 

 during his migrations over the globe. 



With respect to cultivated plants, we shall here take under con- 

 sideration only cases of their naturalization and subsequent reversion 

 to conditions of an independent existence, to a so-called "wild" state, 

 becoming what are known as "escapes". The reversion of cultivated 

 plants to a wild state may take place in various ways. In some cases 

 the plants continue to grow only in the same place where they were 

 formerly cultivated. Such plants are known as anthropogenous relics. 

 In other cases they may, as a result of various natural factors, become 

 distributed beyond the Hmits of the place where they were once culti- 

 vated. This has from ancient times not infrequently taken place, 

 particularly in localities where the natural vegetation has previously 

 been destroyed and habitat conditions disturbed by man. However, 

 the number of such plants, for reasons which we shall take up later, 

 is not very large. 



102 



