242 PLANTS, SEEDS, AND CURRENTS 



maritima are associated with the pyrenes of Tournefortia gnaphalodes, 

 the seeds of Ipomcea, the " stones " of Sccevola Plumieri, and well- 

 rounded small pumice pebbles, 5 to 12 mm. in size. 



These seeds, as I have said, could readily be carried in the crevices 

 of floating logs, or in the cavities of floating pumice, such as is 

 stranded on the beaches of tropical regions all over the world. But 

 it is on their great floating powers, which fit them for dispersal by 

 currents, that we must mainly rely. Neither the kernel nor its 

 hard covering has any buoyancy, the floating power arising, as also 

 ascertained by Schimper (p. 163), from the unfilled space in the seed- 

 cavity. In my paper on the plants of Keeling Atoll I refer to some 

 experiments there made which only indicated a capacity of floating 

 between two and six days in sea- water ; but as the seeds are described 

 as rather soft, it is evident that they were immature. Schimper in 

 an experiment made at Bonn (p. 165) kept the seeds afloat in salt- 

 water for nearly five months (143 days) ; and my experiments in 

 Jamaica and the Turks Islands point in the same direction. Thus 

 in Jamaica some seeds which had been floating in sea- water for seven 

 weeks were quite sound at the close of the experiment. In the 

 Turks Islands I placed sixty seeds in sea- water and after nine weeks 

 forty-five were floating buoyantly, and would evidently have floated 

 for a much longer period. Of the seeds that sank nearly all were 

 empty; whilst of those that remained afloat nearly all had dry 

 sound kernels and dry cavities. 



Swietenia mahagoni, Jacq. (Mahogany) 



In one's inability to explain its mode of dispersal over the West 

 Indian area and the mainland of tropical America, this tree must be 

 typical of many other trees of the forests of this region. Having 

 made a special study of the fruit in Jamaica, the results of which 

 are given in my work on Seeds and Fruits, I here give a few remarks 

 on the plant from the standpoint of dispersal. 



Belonging to a genus of only three or four species that are restricted 

 to the tropics of the New World, this tree does not raise awkward 

 questions, such as are presented by genera common to the eastern 

 and western hemispheres. Yet queries almost as difficult to answer 

 are implied in its occurrence in the larger West Indian islands. 

 Its distribution in South Florida, Mexico, Central America, and Peru 

 may be a matter of the continuity of the land-surface, and there is 

 much, as far as the plants of the Greater Antilles are concerned, 

 to support the contention of the geologist that with those large islands 

 in past ages distribution was also a matter of the continuity of the 

 land-surface. The occurrence of the Mahogany tree in the Greater 

 Antilles and in most of the larger islands of the Bahamas suggests 

 questions that are concerned with former continental connections 

 rather than with means of dispersal. 



The large winged seeds, two and a quarter inches in length, that 

 are freed by the dehiscence of the capsule, are quite unfitted for 

 transport by the currents. In experiments they can float a week 

 or two; but they absorb sea- water and become sodden and dead, 



