E. V. Wulff —120— Historical Plant Geography 



Water as a Factor in the Dispersal of Plants: — Adaptations for the 

 dispersal of plants by water are possessed chiefly by plants living in or 

 near water; nevertheless, due to the ability to float of most fruits, 

 seeds, and other parts of plants, it is quite probable that they may be 

 dispersed by water. We should, however, draw a clear distinction be- 

 tween the dispersal of aquatic and terrestrial plants. As regards the 

 former, their dispersal by means of water— sea-currents, rivers, etc.— 

 is obvious and does not require any special proof. With respect to 

 terrestrial plants, however, we can only assume the possibility of their 

 dispersal by sea and river currents, floating islands, ice-cakes, etc. We 

 shall consider each of these modes of dispersal separately. 



I. Sea-Currents. ~ As early as 1605 CLUsros and, somewhat later 

 (1696), Sloane published papers, in which they directed attention to 

 fruits borne by Atlantic Ocean currents to the shores of Scotland, 

 Ireland, and the Orkney Islands. Linneaus wrote about fruits and 

 seeds of plants borne by the Gulf Stream from the American tropics 

 (apparently from the Antilles) to the shores of Norway. Since those 

 times numerous authors have cited analogous facts of the dispersal of 

 seeds, fruits, and other parts of plants, even floating trees, but we need 

 not enumerate them here (for further details see Hemsley, 1885, 

 Ridley, 1930, and others). 



Nevertheless, the question as to whether or not sea-dispersed seeds 

 can germinate, establish themselves, and serve for the further dispersal 

 of the plants was at first scarcely touched upon. One of the first to 

 give it serious attention was Darwin. He made a number of experi- 

 ments which showed that seeds of many plants after prolonged im- 

 mersion in sea water preserve their ability to germinate. Despite the 

 favorable data that he obtained from these experiments (which, it 

 should be noted, were quite primitive and, hence, not very convincing), 

 his conclusions on this point are far from indicating definite conviction 

 even on his part. As he puts it, "The floras of distant continents 

 would not by such means become mingled; but would remain as 

 distinct as they now are" ("Origin of Species", 6th ed., 1911, p. 514)- 

 Interesting experiments were also made by G. Thuret (cited by 

 DE Candolle) on the ability of seeds to preserve their germinating 

 power after prolonged immersion in sea water. Out of the many seeds 

 that he tested only those of ten plants survived and preserved their 

 ability to germinate. Thuret, and also de Candolle, came to the con- 

 clusion that, with the exception of plants accustomed to grow on sandy 

 shores, it is diflicult to presume that seeds borne by sea currents could 

 find suitable conditions for their development and multiplication. 



Hemsley (1885, p. 42) fists the seeds of over 120 species of plants 

 (belonging to 48 families) dispersed by sea currents. In particular, 

 with respect to the Bermuda Islands, he considers that 45 species of 

 their flora are of such origin. But, despite the undoubted significance 

 of sea currents in the dissemination of seeds, especially as regards the 

 stocking of islands devoid of vegetation, such as coral islands, there are 

 facts showing that in some cases these currents have not played the 

 r61e as a factor in the geographical distribution of plants attributed to 

 them. As an example we may take the coco palm, a typical Uttoral 

 plant. Its wide range within the tropical zone and the ability of its 



