94 



PLANTS, SEEDS, AND CURRENTS 



plants would be able to withstand the six months' passage, as far 

 as the buoyancy and soundness of their seeds and fruits are con- 

 cerned. The implication is that the Counter Equatorial Current is 

 ineffective for this purpose, and if it has not enabled these four 

 plants to cross the Atlantic and pass the barrier that divides them 

 from West Africa, it could have offered no opportunity to other 

 American beach plants, such as Coccoloba uvifera, etc., that are far 

 less suited for dispersal by currents. 



Summary 



1. Before dealing with the similarity between the West Indian 

 and West African littoral floras, a subject intimately bound up with 

 the dispersal of plants by currents, the ground is cleared by a brief 

 discussion of the discontinuity of tropical genera, with the object 

 of showing that whilst we can only appeal to the currents in the 

 case of the littoral species, the distribution of the genus around the 

 tropics often raises other issues (pp. 83-84). 



2. The important conclusions arrived at in the previous chapter 

 that the Main Equatorial Current offers a ready and rapid means 

 of transport for fruits and seeds from tropical West Africa to the 

 New World, and that the opportunity of their reaching West Africa 

 in a sound condition from the tropics of the New World is very 

 slight, are here utilised. Except in the case of the small-seeded 

 beach plants, where other questions than those of dispersal by 

 currents are concerned, the test of fitness for transport in the Main 

 Equatorial Current from West Africa to Brazil is now applied to 

 the littoral, estuarine and riverside plants of the West Indies. In 

 answer to the question whether or not they respond to the test, 

 the following results have been obtained (pp. 84-88). 



3. Taking the plants in the mass, nearly 90 per cent, of those found 

 in the tropics of both the Old and the New World and only 24 per 

 cent, of those confined to the New World possess seeds or fruits that 

 would be able to undergo this trial (pp. 89). 



4. But this testimony in favour of the currents is not equally 

 provided by the plants when regarded from their different stations. 

 Whilst the mangroves with their usual associates and the numerous 

 cosmopolitan beach plants, all of them typical of both sides of the 

 Atlantic, respond almost without exception to the test, the majority 

 of the beach plants restricted to the New World (seven out of ten) 

 do not comply with it. Similar direct and indirect testimony on 

 behalf of the currents is supplied by the plants that grow often at 

 the riverside, especially in the wooded districts above the estuary. 

 Here the purely New World types are as a rule quite unfitted for 

 effective oceanic dispersal, while those found on both the American 

 and the African sides are in most cases adapted for it (pp. 89-94). 



5. On the other hand, the evidence of the six estuarine plants points 

 in the opposite direction, since the only one that occurs outside the 

 New World is quite unsuited for the ocean traverse, while one of 

 those that are peculiarly American is well fitted for it (pp. 90-91). 



6. It will be therefore observed that although on the whole the 



