Chapter VII — 113— Artificial Factors 



Geographical Limits to the Distribution of Exotics: — There are a 

 number of exotics seemingly unaffected by the above-described ob- 

 stacles to their dispersal, there apparently being no limit to their 

 spread over the globe. Among such plants found in Europe, seemingly 

 independent of the laws of the geographical distribution of plants, we 

 may mention: Elodea canadensis, Amaranthus retroflexus, Impatiens 

 paroiflora, Veronica Tournefortii, V. polita, V. agrestis, Linaria cym- 

 balaria, Solidago serolina, Erigeron canadensis, Xanthium spinosum, 

 Galinsoga paruiflora, and several American species of asters. But more 

 thorough and detailed study shows that even in the case of such 

 cosmopolitan species, despite the exceptionally wide range of their 

 adaptability, their distribution has its geographical limits. 



Dr. A. I. M.4LTSEV, in his monograph on the weeds of the U.S.S.R. 

 (1926), has assembled considerable data on the geographical distribu- 

 tion of weeds and exotics. On the basis of many years of investigation, 

 Dr. Maltsev concludes that only a very few weed species, such as 

 Agropyrum repens, Capsella bursa- pastoris, Chenopodium album, and 

 Taraxacum officinale, have extensive areas of distribution, these species 

 being characterized by numerous forms and races adapted to diverse 

 habitat conditions. In the case, for instance, of Taraxacum officinale, 

 a monographic study of the species has shown it to be a coenospecies 

 and has shown the need of dividing it not only into various races but 

 into several, separate species. It may be that this will also be found 

 to hold true for the other species we have just enumerated. 



Most weeds have more limited areas of distribution, beyond whose 

 bounds they cannot pass. Thus, wild oats, Avena fatua ssp. septentrio- 

 nalis, is distributed only in the northern part of European U.S.S.R., 

 while in the south it is replaced by another form, Avena fatua ssp. 

 fatua. Many weeds are confined chiefly to the Black Soil zone, beyond 

 whose limits they rarely extend {e.g., Allium rotundum, Falcaria Rivini, 

 Vaccaria parvijlora, and others) ; outside of this zone they appear again 

 only in the Crimea and the Caucasus. Very common weeds in central 

 regions of the U.S.S.R., such as Cirsium arvense, Sonchus arvensis, and 

 Agrostemma githago, are replaced in the southeast by entirely different 

 weeds. The extreme south has its own weeds, such as Centaurea picris, 

 various species of Caucalis, etc. Weeds of the forest zone in passing 

 to steppe regions often lose their weed character. Some weeds, such as 

 Centaurea cyanus, known to ancient writers, on spreading south, even 

 show a break in their distribution, disappearing in the arid, steppe 

 zone and reappearing in the Crimea and the Caucasus (Pachosky, 

 1911; KoROLEVA, 1930). 



Conclusions: — The significance of man in changing the vegetation 

 of the earth, particularly as regards disturbing its natural state, is un- 

 questionably immense; his role in the distribution of plants over the 

 globe is, no doubt, very considerable, but it is, nevertheless, greatly 

 limited by the specific habitat conditions required by each particular 

 species of plant. The significance of man's role with respect to the 

 present distribution of species throughout the world has been greatly 

 exaggerated, and it is only during the past few centuries that man's 

 role" has acquired the importance that it has. Only the comparatively 



