Chapter V —71— Types of Areas 



new forms as independent species. In a number of cases such new 

 forms were given specific rank on the sole grounds of a morphological 

 study and the estabUshment of the existence of independent areas, and 

 it was only later ascertained that these new species were polyploid 

 forms. In other cases the establishment of the polyploidy of forms 

 morphologically differing but slightly from the type has explained much 

 that was incomprehensible in their geographical distribution. 



Chromosome doubUng is accompanied not only by changes in 

 morphological structure but also by changes in the biological properties 

 of a plant, which likewise play an important role in determining the 

 geographical distribution of these new species and forms. The distri- 

 bution of plants, the fact that they have clearly defined areas, is 

 determined by ecological factors now existent or prevaiUng during 

 earlier periods in the history of the given species. Only a few species 

 of plants possess such a wide range of adaptability to extreme ecological 

 conditions as to enable them to become distributed over practically the 

 entire globe and as to serve as grounds — although not without strain- 

 ing the point somewhat, as we have seen — to call them cosmopolitan. 

 Other species possess a narrower ecological adaptability, which results 

 in their areas of distribution being more localized. In the latter case 

 it is clear that, if for one or another reason changes occur in the 

 biological properties of a species, its dependence on ecological factors 

 is likewise changed, and this causes changes in its area of distribution, 

 which may now extend beyond those boundaries which set a limit to 

 the spread of the initial species. The occurrence of polyploid races is 

 usually accompanied by such phenomena, involving changes in geo- 

 graphical distribution. 



The significance of these changes for the geographical distribution 

 of plants is quite apparent. This is shown by the considerably greater 

 ability of the new form to tolerate habitat conditions unusual for the 

 initial form, such as low or high temperature, inadequate moisture, 

 specific soil conditions, etc. Or, to put this in sequence of time: such 

 conditions are the cause of the origin by mutation of a polyploid form 

 adapted to the given extreme habitat conditions; here there arises the 

 center of a new area, from which proceeds the dispersal of the new form 

 over a territory frequently adjoining the area of the initial species but, 

 due to its different ecological conditions, inaccessible to the latter. 



Nevertheless, it would be erroneous to consider that only floras of 

 regions characterized by extremes as regards ecological conditions em- 

 brace polyploid species and forms. There is no doubt that micro- 

 climatic conditions, micro-relief, and local edaphic conditions may also 

 cause such mutant forms to appear. A study of the floras of the globe 

 from this point of view should supply an answer to these questions. 

 Work in this direction has only just begun, but, nevertheless, in the 

 case of a number of species, there have already been published data- of 

 great interest for botanical geography. 



If we take, for instance, the genus Alopecurus, comprising a poly- 

 ploid series of species with chromosome numbers ranging from 14 to 

 108, we find that the alpine and arctic species have the largest chromo- 

 some numbers, which indicates that new polyploid species arose during 

 the dispersal of this genus into mountainous and arctic regions (Strel- 



