MISCELLANEOUS PLANTS 



235 



But only half of the survivors proved to have been quite impervious 

 to water. In the other half, water had begun to penetrate into the 

 empty cavity as well as into the cavity containing the seed ; and as 

 they all sank in fresh-water the limits of their floating powers had 

 evidently been nearly reached. The upshot of the experiments was 

 that only a third of the stones were in a sound germinable condition 

 after eighteen weeks' flotation in sea- water ; and I would imagine that 

 a period of five to six months would represent their flotation capacity. 



On the whole the results of experiments on the capacity for dis- 

 persal by currents possessed by Sccevola Plumieri and Sc. Kcenigii 

 go to show that although in both cases the stones will float unharmed 

 for months in the sea, the advantage is certainly on the side of the 

 plant last named. We would expect that floating capacity dependent 

 on the existence of buoyant tissue in the fruit-coverings would be 

 more effective than when determined by the failure of seeds and the 

 resulting empty cavity. Whilst the stones of Sccevola Kcenigii will 

 float in sea- water for a year or more, those of Sc. Plumieri will on the 

 average float in a sound condition for only four or five months. 

 Those of the first-named species could very well be drifted across an 

 ocean as broad as the North Atlantic; but this would not be prac- 

 ticable for those of Sc. Plumieri, their floating powers only allowing 

 them to reach Bermuda from the Florida coasts, the transatlantic 

 traverse, occupying twelve months and more, being impracticable. 

 But although the transference of Sc. Plumieri from the New World 

 to West Africa by the only available route in the Gulf Stream drift 

 would be impossible, the passage from tropical West Africa to Brazil 

 in the Main Equatorial Current would be quite within the floating 

 capacity of its fruits, since, as shown in Chapter III., it could be 

 performed in three months. I have not here referred to the possi- 

 bility of a passage to West Africa in the Counter Equatorial Current, 

 since there is little to indicate that it is an available route for seed 

 dispersal. 



When, therefore, we discuss from the standpoint of dispersal by 

 currents the question whether Sccevola Plumieri has reached the 

 West Coast of Africa from America, or the Atlantic coasts of America 

 from Africa, we have to exclude the long easterly passage in the 

 Gulf Stream drift in preference for the westerly passage in the swift 

 Main Equatorial Current from the Gulf of Guinea to the coasts of 

 Brazil. From the standpoint of distribution a consideration of the 

 same question leads to the same results. There is nO probability 

 that the American region could have received this plant from across 

 the Pacific. Although Australia is the home of the genus, Sc. 

 Plumieri does not occur there, or, in fact, anywhere in that region 

 of the globe. We have, therefore, to choose between America as 

 the birthplace of the species or America as its recipient from the 

 African West Coast. Although in the Index Kewensis America is 

 credited with two endemic species of Sccevola, one in Trinidad and 

 the other in Central America, these are both disregarded by Krause, 

 who includes the species of the second locality (Sc. cumana) amongst 

 his doubtful species (p. 168). No species of the genus is mentioned 

 in Hart's list of the Trinidad flora. There is, therefore, little reason 



