74 PLANT GEOGRAPHY 



CHAPTER VI 



BARRIERS 



THE number of seeds produced by many species is so 

 great that, if not prevented, any one of them might 

 cover the land of the globe in a very few years. The 

 Henbane, for instance, produces about 10,000 seeds a 

 year, which, it has been calculated, would lead to the 

 covering of the land within five years ; whilst the average 

 number of seeds of the Tobacco is thirty-six times greater. 

 Not only, however, are there many risks attendant upon 

 seeds and seedlings leading to their early destruction, 

 but the preoccupation of the ground limits their chances 

 of sprouting, and they may only retain their germinating 

 power for a short time. " As we have seen, moreover, 

 species vary much in their mobility or facility of 

 dispersal. 



The main divisions of the Vegetable Kingdom occur in 

 all regions of the earth's surface, though one Class, the 

 CytadecB, is, at the present day, confined to the Southern 

 Hemisphere. Several of the larger Natural Orders, such 

 as Composites, Leguminosce, Cyperacea, and Graminacecz, 

 also occur in all regions, though in very varying propor- 

 tions. Leguminosce diminish with increasing cold, Com- 

 posites with increasing cold and moisture, and Grasses 

 with increasing dryness of climate. Some few genera, 

 such as Senecio, Rubus, Plantago, and Oxalis, are cosmo- 

 politan, i.e. have representatives in all regions, the first 

 named being noteworthy in that its thousand species 

 present adaptations to almost every possible condition 

 of climate or habitat, some semi-aquatic, many " weeds " 

 of waste-lands (" ruderal ") or woodlands, ivy-like 

 climbers, succulent almost leafless forms, or shrubs, or 

 trees with very large leaves. Though some aquatic 

 species and herbaceous weeds of cultivated ground or of 

 waste places are very widely distributed, few are even 

 approximately cosmopolitan. The Shepherd 's-purse 

 (Capsella Bursa-pastoris Med. Pflan.) is nearly so. 

 Plantago maritima L. occurs on our European shores, 

 in Cape Colony, and at the southern extremity of 

 America; and many Iceland species are found also in 

 the Falkland Islands. 



