WEST INDIAN AND WEST AFRICAN FLORAS 93 



of dispersal at all. A curious question is raised in the case of 

 Conocarpus erectus, the small seed-like fruits of which possess con- 

 siderable floating powers. It grows on the Pacific coasts of tropical 

 America, and its absence from the Pacific islands is remarkable, 

 since there seems to be no reason, as far as its capacity for dispersal 

 by currents is concerned, why it should not have reached the eastern 

 islands of the tropical Pacific. The same difficulty is presented by 

 the Manchineel tree (Hippomane mancinella), a matter which is 

 referred to below. When dealing in detail with Conocarpus erectus 

 it will be pointed out that Schimper regarded it as a current-dispersed 

 plant. On the whole it may be considered that the three beach 

 plants of tropical America that are only known outside the New 

 World from the shores of West Africa raise many difficulties. 



The Beach Plants confined to the New World. — On the other hand, 

 when we come to the West Indian beach plants that have not been 

 found outside the New World we learn that of the ten plants named 

 in the table the majority do not respond to the test. Whilst most 

 of them possess fruits or seeds that can only float for a few days or 

 weeks, in three cases, those of Hippomane mancinella (the Manchineel 

 tree), Tournefortia gnaphalodes, and Morinda royoc, the floating 

 capacity covers several months and may extend to a year or more. 



To illustrate this point I will take the case of the Manchineel 

 tree, the fruits of which are one of the most common constituents 

 of beach-drift in the West Indian region ; and there can be no doubt, 

 as shown on a later page, that they can float for many months in 

 the sea with their seeds uninjured. It is a tree that has held its 

 present area for ages. In the hammocks of the Florida everglades 

 there have found a sanctuary many West Indian plants that flourished 

 in the same locality when the hammock-lands represented an ancient 

 system of keys or low islands that rose from the waters at the time 

 that the Gulf Stream flowed across the present peninsula of Florida 

 (Harshberger, p. 230). The Manchineel was one of them. For a 

 long period currents have afforded considerable opportunities for 

 the transference of littoral plants from West Africa to the New 

 World, but very few for a transference from America to Africa. 

 Yet it may be argued that since the Manchineel grows on the beaches 

 of the Pacific coast of the New World its fruits could have reached 

 the Old World across the Pacific. However, the complete traverse 

 of the ocean would occupy a period of two or three years, a subject 

 discussed in Chapter XIII. Yet only about half that time would be 

 required for its fruits to reach the Paumotuan, Tahitian, and Mar- 

 quesan groups in mid-Pacific, and it is difficult to understand why 

 the tree has not been recorded from the beaches of the Pacific islands. 



It is still more difficult to understand, in the case of the tropical 

 Atlantic, why, if the Counter Equatorial Current is an effective 

 agency in seed dispersal, beach plants like the Manchineel, Tourne- 

 fortia gnaphalodes, and Morinda royoc have not reached West Africa 

 from the New World, and we may say the same of Sacoglottis amazonica 

 among the estuarine plants. The time required for this traverse 

 would be about double that needed for the traverse in the Main 

 Equatorial Current in the opposite direction, and all these American 



