E. V. Wulff —130— Historical Plant Geography 



others by the humidity of the air (some plants cannot withstand a dry 

 atmosphere and perish even when there is adequate soil moisture; 

 others cannot endure a humid atmosphere). Hence, a plant acciden- 

 tally brought into a region characterized by climatic conditions unsuited 

 to it will be unable to develop and will be doomed to perish, even if its 

 seeds germinate. 



3. Edaphic Barriers. — What we have just said about climatic 

 conditions holds true for other habitat conditions, particularly edaphic 

 conditions, such as the physical and chemical structure of the soil, soil 

 humidity and temperature, etc., which in combination or separately 

 may constitute a barrier to the spread of a species or limit its range to 

 only those localities characterized by certain definite conditions. 



The foregoing, however, are not all the difficulties that a species 

 must overcome in its conquest of new territories. Synecology, or the 

 science of the interrelations of plants in plant communities, has shown 

 us what great significance such interrelations have in the distribution 

 of plants. As Braun-Blanquet (1928) has emphasized, the chief 

 difference between plant and animal communities is that the former 

 are characterized by a struggle for existence not limited by any prin- 

 ciples of usefulness, division of labor, communal toil, or by any other 

 factors arising from the conscious activity of organisms forming part 

 of a biocoenosis. 



This struggle for existence, or competition between separate species 

 and individual plants, is comparatively insignificant in so-called "open 

 associations", on sands, cliffs, and solonchak soils, where the vegetation 

 does not form a continuous carpet but is spotted, with larger or smaller 

 unoccupied spaces between the plants. But here habitat conditions are 

 usually so specific and unfavorable that only a few species are adapted 

 to them. In other localities, where edaphic and physiographic condi- 

 tions are more favorable, the vegetation forms "closed associations", 

 the members of which utilize every bit of soil, every ray of sun. In 

 such associations competition between the plants, with respect to space, 

 light, and food, constitutes a very characteristic feature. There is no 

 call for us here to enter into a discussion of the problem of the struggle 

 for existence in the plant world. If we have touched upon this prob- 

 lem, it has been only with the aim of pointing out that even among 

 the members of a given association, as a result of natural selection and 

 the struggle for existence, there survive only an insignificant percentage 

 of the progeny, and that, consequently, the chances of a new, would-be 

 member that happened to spring up there from an accidentally intro- 

 duced seed to gain full citizenship rights in such a closed association 

 are almost nil. 



The foregoing facts make it necessary to regard quite skeptically 

 the statements of old authors who but slightly delved into these 

 problems in plant life (phytosociology) and also those of modern authors 

 who exaggerate the significance of chance factors in the distribution of 

 plants over the globe. We believe that we have made it sufficiently 

 clear that it is practically impossible for an accidentally introduced 

 seed to give life to a plant that can enter into the composition of an 

 alien association. In order for a plant to become established in an 

 entirely new region, special conditions are necessary, viz., territory suit- 

 able for plant life but unoccupied or not fuUy occupied by other plants. 



