XXXII THE PANAMA ISTHMUS 



499 



afloat in sea-water for five weeks, and no doubt they will float for 

 months. 



The seed-drift to be observed stranded on the beaches and 

 floating in the estuaries on both sides of the isthmus is, generally- 

 speaking, the same — a circumstance of great importance in plant- 

 distribution, since we can here see rivers bringing down the same 

 seeds from the same " divide " to the shores of the Pacific and 

 Atlantic oceans. In the case of a plant like Entada scandens, 

 which grows in the interior, this is a matter of much interest, as it 

 thus possesses here a centre of dispersal from which its seeds can 

 be carried by the currents eastward to the West African coast 

 and westward across the Pacific to Malaya and (given time) 

 around the shores of the Indian Ocean to the East African coast. 

 In describing the possible routes of dispersion from this centre I 

 have described the distribution of the species. 



I am indebted to Mr. Holland, of the Kew Museum, for the 

 identification of some of the drift-seeds and fruits collected by me 

 on the isthmus, those identified by him being followed by the 

 letter H. On the beaches and floating in the estuaries on both 

 sides of the isthmus I found Rhizophora seedlings ; seeds of 

 Entada scandens and Mucuna urens (medic), H. ; seedvessels of 

 Spondias lutea (Linn.), H. ; Prioria copaifera (Griseb.), H., with 

 decayed seed ; and the empty nuts, i^ to 2 inches in size, of more 

 than one species of Astrocaryum, H. Although in the case of the 

 two last-named genera the seedvessels were useless for dispersal, 

 being evidently brought down from the interior by the rivers, 

 they serve to illustrate the important principle that the rivers 

 bring down the same seed-drift on both the Atlantic and Pacific 

 coasts of Central America. Mr. Hemsley includes amongst the 

 seed-drift stranded on the coast of Jamaica the seedv^essels of 

 Spondias (probably S. lutea) and of Astrocaryum (Bot Chall. 

 Exped., iv. 299, 304). 



Those of Spondias lutea were found by me floating in the 

 Guayaquil River and stranded on the beaches of Ecuador and of 

 the Pacific and Atlantic coasts of the Panama isthmus. This is 

 the Hog-plum, which in tropical America and the West Indies is 

 both wild and cultivated. Its buoyant "stone" has a covering of 

 cork-like air-bearing tissue. This is a remarkable case of non- 

 adaptation in the matter of buoyancy. The seedvessels cut across 

 contained sound seeds ; and they are provided with the essential 

 qualities of " long floaters." 



K K 2 



