E. V. Wulff —126— Historical Plant Geography 



BiRGER (1907) investigated 170 birds of more than 35 species with 

 the aim of finding seeds or small cakes of earth containing seeds ad- 

 hering to the surface of their bodies (feet, legs, feathers), but he did 

 not find a single case of such adhesion. As regards the endozoic dis- 

 persal of seeds by birds, he found in the stomachs of the birds viable 

 seeds of over 40 species of plants, but he was not inclined to ascribe 

 great significance to this mode of seed dispersal because of the very 

 short time that seeds remain in the digestive tract of a bird. For this 

 conclusion he found further confirmation in the fact that the percentage 

 of species with fleshy fruits in the flora, for instance, of Sweden is very 

 insignificant. If one leaves Rosa and Ruhus out of consideration, 

 species of the latter being concentrated only in southern Sweden, the 

 number of species with fleshy fruits in the flora of Sweden will be only 

 4.3 per cent. The percentage is somewhat higher, if one includes 

 fleshy-fruited epiphytes growing in Sweden and Norway and also on 

 islands that have recently appeared due to a lowering of the level of 

 the sea. For instance, Birger found that on an island, which had 

 existed as such for only four years, out of 112 flowering plants there 

 were only 5 species with baccate or drupaceous fruits, but on an island 

 that had appeared above the level of the water ten years previously, 

 out of 177 flowering plants there were 7 such species and a 22-year-old 

 island out of 191 flowering plants there were 18 species with edible 

 fruits. From these data Birger arrives at the conclusion that, in the 

 case of newly stocked territory, species with fleshy fruits form a con- 

 siderable percentage and this may be ascribed to the role of birds in the 

 introduction of seeds. 



Unfortunately, however, there has not been reported a smgle actual 

 observation of the transport by birds of the seeds of any plant that 

 subsequently succeeded in establishing itself and extending its range m 

 the new habitat. All the available data with respect to this as to 

 other kinds of plant dispersal by natural factors have the defect that 

 they cover only separate phases, only separate links in the process of 

 seed dispersal. Thus, we know that some seeds pass through the 

 intestines of birds uninjured, retaining their germinating power; we 

 know, in some cases, the length of time seeds remain in a bird's di- 

 gestive tract; we know the rate and direction of flight of many birds- 

 yet no one has traced, nor could have traced, the fate of seeds de- 

 voured by birds before a flight. Consequently, all the proofs pro and 

 con bear a purely conjectural and very superficial character. The 

 only thing that we can affirm with assurance is that, as regards the 

 seeds of a very few plants, chiefly swamp and littoral plants, it may be 

 presumed that they might be carried by birds to a new locality and 

 take root there. 



Significance of Natural Factors in the Distribution of Plants: — 



Having discussed each of the principal natural factors in plant dis- 

 persal, we may now pass to a consideration of the problem as a whole 

 and attempt to find an answer to the question: Can natural factors in 

 plant dispersal explain all the peculiarities in the present distribution of 

 plants over the earth's surface, or are there other causes that have induced 

 discontinuities in their distribution? 



