XVII 



C^SALPINIA 195 



buoyant seeds of C. bonduc was that the two kinds of cavities were 

 more often combined. 



The reason of the absence of floating power was clearly 

 indicated in the non-buoyant Hawaiian seeds, where there was no 

 central cavity, or it was represented by a narrow slit. The solitary 

 buoyant seed found in the beach drift had a typical large central 

 cavity. With the non-buoyant seeds of the inland species of the 

 mountains of Vanua Levu it was ascertained that two-thirds had 

 loose kernels with the cotyledons closely appressed. In the others 

 there was a lateral cavity outside the kernel, the central cavity 

 being only represented by a slit, a hair's width in breadth. In 

 the non-buoyant seeds of C. bonduc, the central cavity was only 

 2 to 3 mm. wide, and the lateral cavities were small. 



Respecting the influence of " station " in producing the 

 differences in buoyancy, it cannot be said to be connected with the 

 maturation of the seeds of inland plants under more humid con- 

 ditions than those which prevail at the coast. In Fiji some of the 

 littoral plants with buoyant seeds grow on the mangrove-trees in 

 the shade and humidity of the swamps ; whilst in Hawaii the 

 inland plants of Caesalpinia bonducella with their non-buoyant 

 seeds thrive in exposed arid situations in districts of little rainfall, 

 such as on scantily vegetated lava-flows. With non-buoyant seeds, 

 where there is little or no cavity, the cotyledons are always 

 thicker and moister than in the case of the seeds that float. 

 Though associated with differences in station, as implied in the 

 terms " coast " and " inland," the cause of the difference in 

 buoyancy is not connected with different degrees of humidity, but 

 with some other cause or causes acting on the spot which, while 

 they favour the drying of the kernel in coast plants before the 

 seed-coats finally set, impede it in the inland plants. That the 

 seed does not subsequently acquire floating power, even after 

 years of drying, was shown in several of my experiments. 



The light, unopened prickly pods of both species float buoyantly, 

 even when the inclosed seeds have no floating power. In an 

 experiment on Caisalpinia bonduc in Fiji the pods remained afloat 

 after a month in sea-water. With those of C. bonducella in Hawaii 

 I found that they floated for several weeks, and in one case a pod 

 was afloat after three months. The pods dehisce on the plant ; 

 but they sometimes do not open sufficiently to allow the seeds to 

 fall out. The pods, however, have to be torn off from the plant, 

 and are not likely to occur in the drift. Indeed, they never came 

 under my notice in any locality in the drift, and as an effective aid 



O 2 



