176 PLANTS, SEEDS, AND CURRENTS 



stated to be partial to these fruits, one species (Anona palustris) 

 being known in Jamaica as the Alligator Apple. 



Since none of the Anona seeds examined by me are impervious to 

 water, it is unlikely that they would be able to withstand the injurious 

 effect of the digestive fluids of birds for more than a few hours. It 

 is quite possible that a pigeon might transport them unharmed 

 across a strait two hundred miles in width, but that would be the 

 limit. We must look for other agencies, therefore, to explain the 

 case or cases where broad tracts of ocean have presumably been 

 traversed by the species ; and when the fruit of such a plant is inedible 

 and the species is not likely to have been spread by man we must 

 appeal to the currents. If in addition to these features we find that 

 the plant concerned is a denizen of coast swamps and is associated 

 with mangroves that are well known to be distributed by currents, 

 then the presumption in favour of the current hypothesis becomes 

 very strong, and we would expect to find that the seeds of all other 

 species, where the question of oversea transport is scarcely raised, 

 would be ill-fitted for crossing an ocean in a current. It is not a 

 matter of fruit-buoyancy, since the floating fruit would soon break 

 down, as is illustrated in the case of that of Anona palustris in a 

 later page, and the fate of the species would be ultimately determined 

 by the behaviour of the liberated seeds. 



What, therefore, we may ask, is the behaviour of seeds of Anona, 

 when their buoyancy is tested? The answer to this question is 

 indicated on p. 177 ; and though the seeds of only five species have 

 been experimented upon, the data go to show that Anona comes 

 into line with many other tropical genera where only the littoral 

 species are adapted for dispersal by currents. 



It will be learned from the results tabulated on p. 177 that it is only 

 the littoral species with inedible fruits and seeds capable of floating 

 a long time in sea- water that occurs outside the New World, namely, 

 on the coast of tropical West Africa. This species, Anona palustris 

 (syn. A. glabra), deserves a special discussion. 



We may best begin with Dr. Safford's letter to Prof. Harshberger, 

 which is given on p. 155 of the latter' s paper on the vegetation of 

 South Florida (Trans. Wagner Free Institute of Science, Philadelphia, 

 October 1914) : 44 A letter of query directed to Dr. W. E. Safford, 

 who has monographed the genus Annona, as to the means of dis- 

 tribution of the custard-apple, elicited the following information under 

 date of August 20, 1912 "— 



" Dear Sir, 



44 1 have the honour to acknowledge the receipt of your 

 recent letter in which you ask suggestions as to how the swamp- 

 apple, Annona glabra, is dispersed. This is a question which has 

 perplexed me. The associations of this plant are with species of 

 so-called mangroves of wide distribution. This species itself occurs 

 on both sides of tropical America, the Galapagos Islands, and the 

 West Coast of Africa. It would be interesting to find out whether 

 detached branches take root readily in the mud. The wood is so 

 light, it is called corkwood, and the roots are used for corks and 



