E. V. Wulff —122— Historical Plant Geography 



order to explain their present distribution along these shores, he finds it 

 necessary to seek historical causes and to presume the existence, at the 

 time of their invasion of the region, of a different distribution of land 

 and sea and of different climatic conditions than those now prevailing. 

 Ridley (1930) notes that "nearly all sea-dispersed plants belong to 

 genera which have a large number of terrestrial inland species, and that, 

 as a rule, there are only one or two species which are adapted for sea- 

 dispersal" (p. 250). 



2. Rivers and Streams. — The transport of fruits, seeds, and other 

 parts of plants by rivers and streams, flowing from mountains and hills 

 into valleys and plains, is well known. Instances are not uncommon of 

 fruits and seeds that have been borne by rivers for exceedingly great 

 distances — from the mountains down to the very sea. Beguinot (191 2) 

 found, over a territory two kilometers in extent, among the debris 

 washed down to the mouth of the Tiber River cyclamen tubers and 

 seeds of as many as twenty-five different species of plants. 



Rivers undoubtedly also play an important role in the seeding of 

 inundated meadows and, in general, of flood plains at times of high 

 water. Keller (1922), on the basis of a study of the debris left by 

 the Voronezh River (southeast of Moscow) after the spring floods, 

 recorded a considerable number of river-borne fruits and seeds, e.g., 

 the fruits of Aristolochia dematitis, Iris pseudacorus, Rumex crispus, 

 several species of sedges, etc. An anatomical study of these fruits 

 showed that they possessed various adaptations for dispersal by 

 floating. 



3. Glaciers, Icebergs, and Ice-Cakes. — The transport of large stones, 

 boulders, and clumps of earth by glaciers and floating ice-cakes has long 

 been known. This led Lyell to put the question as to the possibility 

 of the chance transport, in this way, of seeds, a mode of transport that 

 might presumably have been of widespread significance during the Ice 

 Age. This view was at first very widely held, it being considered 

 possible, for instance, to explain in this way the stocking of oceanic 

 islands located far from the mainland. 



At the present time, the role of ice as a factor in the geographical 

 distribution of plants is regarded with considerable skepticism, despite 

 the fact that glaciers may, of course, carry seeds from mountains into 

 the valleys at their feet. This was formerly taken as grounds for ex- 

 plaining the occurrence of mountain plants in far-distant valley habi- 

 tats as the result of their seeds having been transported there by 

 glaciers during the Ice Age. The supposition of Christ (1882) that 

 during the Ice Age mosses and certain ferns (e.g., Asplenium septen- 

 trionale) were dispersed by means of erratic boulders, based on the 

 fact that these plants are now found on such boulders, was shown by 

 Amann (1894) to be untenable. The latter investigator studied the 

 various species of mosses growing on such boulders, and he found that 

 they comprise chiefly those species which avoid a lime substratum and 

 which, therefore, have settled on these granite boulders as being the 

 only habitats, due to the limy character of the mountainsides in the 

 Alps and the Juras, that correspond to their biological needs. More- 

 over, the data on the distribution of these species show that among the 

 species dwelling on erratic boulders there is not one real alpine, high- 



