XVII 



ENTADA SCANDENS i8i 



the Guayaquil river, in the estuary of the Chagres at Colon, and in 

 the mouth of a river on the Panama side of the isthmus. 



The mode of liberation of the seeds is worthy of a passing 

 remark. The huge pods, often several feet in length, ultimately 

 break up into separate joints bearing the seeds. The joints may 

 decay on the ground, and the seeds are thus freed ; or not infre- 

 quently in a mangrove-swamp they fall at once into the water, and 

 there they float, as may often be observed in Fijian rivers, until 

 their decay sets free the seed. 



The seeds of Entada scandens are often quoted, and justly so, 

 as striking examples of the dispersal of seeds by currents. Yet in 

 few plants could the nature or the structural cause of the buoyancy 

 have so little claim to be considered as adaptive in its character. 

 Quite half, and sometimes even the majority, of the seeds freshly 

 liberated from the plant have no buoyancy at all. The mean 

 specific weight of the seed is about that of sea-water, but markedly 

 higher than that of fresh water ; whilst the principal determining 

 cause of the buoyancy is, as shown below, purely mechanical, and 

 one that, whilst favouring the wide distribution of the species, could 

 not be improved by or come within the scope of Natural Selection. 



From experiments made in Fiji and Ecuador, it appears that at 

 least 50 per cent., and often more than half, of the seeds when first 

 liberated from the pod have no buoyancy in sea-water. Of those 

 that float in sea-water, a proportion varying between one-third and 

 one-half sink in fresh water, so that in the case of plants growing 

 on the banks of a river only about one-fourth or one-third would 

 be carried down to the sea. So fine is the adjustment of the 

 specific weight of these seeds to the density of water, a subject 

 discussed in its general bearings in Chapter X, that if one gathers 

 a number of drift seeds on a beach, let it be in Fiji or in Ecuador, 

 although, of course, all will float in the sea, only one-half or two- 

 thirds will float in the neighbouring fresh-water stream. Those 

 that float appear to be able to float almost indefinitely. This is 

 sufficiently established by the transport of the seeds in a sound 

 condition by the currents across the Atlantic, and by such evidence 

 as the stranding of seeds incrusted with polyzoa and serpulae on 

 the beaches of Keeling Atoll. It has been also proved by the 

 following experiment. Several years since, I placed a seed in a 

 vessel of sea-water, where it still floated buoyantly in a perfectly 

 sound condition twelve months afterwards. 



With regard to the cause of the buoyancy, investigation shows 

 that neither the seed tests nor the seed contents have any floating 



