Chapter VII —111— Artificial Factors 



pistillate stage they are always closed. This is ascribed to the fact 

 that the climate in Java is more humid than in Mexico, the home of 

 this species, this greater humidity retarding the opening of the flowers 

 by one day. Consequently, at the moment of complete development of 

 the stigmas the flower is still closed and insects cannot penetrate 

 therein, while by the time the flower opens the stigmas are already 

 wilted and have lost their receptivity. 



These two examples sufi&ce to show how varied and precise are the 

 biological adaptations of plants to sundry habitat conditions (very 

 often not perceptible to us), any slight alteration in which may make 

 the plant incapable of a normal, independent existence. If to this we 

 add the unceasing struggle for existence and the competition with the 

 indigenous vegetation, we should not be surprised at the relatively 

 small number of those species introduced by man for cultivation or ac- 

 companying him in his migrations that become fully naturalized com- 

 ponents of the local flora. 



This explains, for instance, the fact that the numerous species 

 dispersed to distant regions by railways very rarely spread beyond the 

 roadbed itself. Thus, Litvinov (1926) reported the finding of southern 

 plants along the Murmansk Railway. Some of them grew there year 

 after year for a considerable number of years, but in no case did they 

 spread beyond the immediate vicinity of the railway Hne and stations. 

 The fact that these southern plants could maintain themselves so far 

 north over a period of years must undoubtedly be accounted for by the 

 presence of sand sprinkled along the roadbed and by the absence of 

 competition, that potent factor in the life of organisms. 



Similarly, the flora of Montpellier has been enriched to only a very 

 slight extent by the numerous species brought in with the wool at 

 Port Juvenal. According to Thellung (1912), the number of species 

 that have been introduced in various ways by man into the flora of 

 Montpellier beginning with the sixteenth century is not over 107, or 

 3.8 per cent of the total number of species in this flora, estimated by 

 him to be 2,792. The smallness of this percentage may be ascribed 

 largely to competition with the local flora and the latter's fuU occupa- 

 tion of the available territory, factors whose importance we have 

 already stressed. 



Adventive plants are ordinarily able to spread only in places where 

 the natural vegetation has been destroyed by man and the habitat 

 conditions to which the plant communities formerly dwelling on this 

 territory had been accustomed have been altered. EspeciaUy favorable 

 conditions are created for the adventive plants, when man, cultivating 

 his land year after year, does not allow the local vegetation to regain 

 its hold. But once such land is left for a time uncultivated, there 

 ensues a struggle between the aliens and the indigenes that usually ends 

 in the victory of the latter. It is precisely the destruction of the in- 

 digenous flora and the marked changes in habitat conditions caused by 

 man that make possible the rapid and widespread distribution of 

 adventive vegetation on islands and on those territories of the mainland 

 newly utilized and colonized by man, for it is in such places that 

 natural plant communities are destroyed prior to cultivation of the 

 land. This holds particularly true in those cases where forests are cut 



