94 A NATURALIST IN THE PACIFIC CHAP. 



It might seem strange that the seeds of Entada scandens should 

 come into the category of seeds with a specific weight near that of 

 fresh water ; yet my observations in Fiji indicate that such is the 

 case. In the discussion of this plant in Chapter XVII. it is pointed 

 out that, as a rule, not more than a fourth will float in a river when 

 they are first freed from the pod, and not more than fifty per cent, 

 will float in the sea. Those that float, however, in either water will 

 usually float indefinitely. The seeds also of Mucuna gigantea D.C. 

 are not very much lighter than fresh water. Out of six seeds that 

 floated in sea-water buoyantly, five floated in fresh water t but heavily. 



It is of interest to notice in this connection that the mangrove- 

 seedlings produced by germination on the tree, as in the case of 

 Rhizophora and Bruguiera, have a mean specific weight somewhere 

 between fresh water and sea-water. This is often illustrated in a 

 curious way, when the seedling has not been prematurely detached 

 from the tree. Thus in the sea off the coast of tropical America, as 

 well as amongst the Fijian Islands, the seedlings of Rhizophora 

 mangle are as a rule to be observed floating horizontally ; whilst in 

 the fresh or brackish water of the estuaries of these regions they 

 assume a more or less vertical position, only the plumular portion 

 protruding above the water. This is also true of the seedlings of 

 Rhizophora mucronata, the Asiatic mangrove, and of Bruguiera 

 rheedii. This subject is discussed in detail in Chapter XXX. ; but 

 it may be here remarked that a good proportion of Rhizophora 

 seedlings, when detached in the mature condition from the tree, 

 have no buoyancy, between 20 and 50 per cent, going to the 

 bottom when they fall into a river, and between 5 and 10 per cent, 

 when they drop into the sea. The navigator might often obtain an 

 indication of the density of the sea-surface when approaching the 

 mouth of a large river by observing the floating Rhizophora 

 seedlings (a foot long) which are carried out to sea in numbers. If 

 he sees them from the deck of his ship floating horizontally he will 

 infer that the surface-water is mainly sea-water. In ordinary fresh 

 water when they float vertically he would not be able to distinguish 

 them from floating seeds or fruits. 



It has only been possible to treat this subject in an illustrative 

 manner. More details might have been given ; but I have gone 

 far enough to bring the following points into relief and to justify 

 one in drawing the conclusions to be now stated. 



Most seeds and seedvessels in respect of their floating powers tend 

 to gather around two centres or means and to form two groups, the 

 sinking group and the buoyant group. 



