92 PLANTS, SEEDS, AND CURRENTS 



of the tropics and subtropics of both hemispheres, those confined to 

 the New World and the west coast of Africa, and those entirely- 

 restricted to the New World. 



The Cosmopolitan Beach Plants. — In the case of this group we 

 are dealing with plants that have travelled in most instances prac- 

 tically around the globe. They include five leguminous species, 

 Canavalia obtusifolia, Guilandina bonduc, G. bonducella, Sophora 

 tomentosa, and Vigna luteola ; two malvaceous species, Hibiscus 

 tiliaceus and Thespesia populnea ; three convolvulaceous species, if 

 we include Ipomoea carnosa with /. pes-capraz and /. tuba ; and six 

 others belonging to as many different orders — namely, Cassytha 

 filiformis, Colubrina asiatica, Dodoncea viscosa, Sc&vola plumieri, 

 Suriana maritima, and Ximenia americana. There are a few of these 

 plants that could scarcely be considered as universally distributed 

 in warm latitudes, but the differences that separate them from the 

 others are only in degree. Thus both Sc&vola plumieri and Vigna 

 luteola occur on both coasts of the New World and on both coasts 

 of Africa; but since the first extends to India and Ceylon and the 

 second exists in Australia, it would be difficult to exclude them. 

 In giving Ipomoea tuba a very wide distribution I have followed 

 Urban, and the reasons in the case of /. carnosa are given in the 

 reference named. 



Every one of these fifteen or sixteen widely spread beach plants 

 has buoyant seeds or fruits (in most cases it is the seed that is con- 

 cerned) which can float for a long time unharmed in the sea; and 

 respecting some, such as the two species of Guilandina and Thespesia 

 populnea, it is known that their seeds can float 'for a year in the 

 sea without loss of the germinative capacity. A plant has, in fact, 

 been raised from a Guilandina seed cast ashore on the coast of Ireland. 

 But there is not a species in this group of beach plants that would 

 not respond to the test involved in the transport of their seeds or 

 fruits in a sound condition from West Africa to Brazil in the Main 

 Equatorial Current, though in the case of Dodon&a viscosa the 

 minimum limit of flotation-capacity is only just attained. 



The Beach Plants restricted to the New World and West Africa. — 

 When the test above mentioned is applied to the small group of 

 beach plants that beyond the New World are only known from the 

 shores of West Africa, we find that two out of the three respond to 

 it — namely, Chrysobalanus icaco and Conocarpus erectus. The third, 

 Ecastaphyllum brownei, presents the same problem that was offered 

 by Andira inermis and Symphonia globulifera amongst the riverside 

 and estuarine plants. Here we have three plants that grow on the 

 opposite sides of the tropical Atlantic with to all appearance no 

 capacity for accomplishing the ocean passage. At first sight it 

 would seem that the validity of the current hypothesis is here im- 

 pugned; but in one case, at least, it is likely that quite other con- 

 siderations arise. Thus Symphonia globulifera, an estuarine tree, 

 belongs to a genus that is mainly confined to Madagascar, the excep- 

 tions being two West African species, one of which, as in the present 

 case, is found in the West Indies. It is obvious that the discon- 

 tinuous distribution here indicated may not be connected with means 



