ii THE FLORAS OF THE PACIFIC ISLANDS 21 



behave very irregularly ; but even here most of the difficulties can 

 be removed, when we come to consider a further extension of the 

 principle. Thus, whilst the seeds of Hibiscus tiliaceus, a wide- 

 ranging littoral tree known to be dispersed by the currents, float for 

 a long time, those of H. Youngianus (Gaud.), an endemic Hawaiian 

 species, and of two wide-ranging species, H. diversifolius (Jacq.) 

 and H. Abelmoschus (L.), also float for some time. The Hawaiian 

 plant, however, grows in wet places ; and this applies also to 

 H. diversifolius which grows in swamps at and near the coast. 

 The extension of the principle to water-side plants generally, 

 which is discussed in the next chapter, will explain the difficulties 

 connected with these two species. But we have in H. Abelmoschus 

 a remarkable exception to any rule of buoyancy, since it grows in 

 dry situations, is often cultivated, and yet possesses a special layer 

 of buoyant tissue in the seed-coats to which the floating power is 

 due. The seeds of Hibiscus esculentus (L.), the widely spread 

 cultivated plant of the tropics, have no buoyancy. 



Some curious indications are supplied by Caesalpinia, a 

 Leguminous genus, containing two wide-ranging shore species. 

 Speaking generally the rule applies ; and I found in Fiji that 

 whilst the seeds of the two littoral plants (C. Bonducella and 

 C. Bonduc) were as a rule buoyant, those of an inland mountain 

 species sank. But it is very remarkable that although the seeds of 

 C. Bonducella have long been known to be transported by the 

 currents, and are often stranded by the Gulf Stream on the coast 

 of Scandinavia, when it grows in Hawaii, where it is as a rule an 

 inland plant, the seeds lose their buoyancy. This is quite in 

 accordance with the general principle ; but I must refer the reader 

 for a general treatment of this genus to Chapter XVII. There 

 also will be found the instance of another Fijian littoral plant, 

 Afzelia bijuga, a common littoral tree with buoyant seeds which 

 also lose their buoyancy when the tree grows inland. A similar 

 instance is afforded by Kleinhovia Hospita, the seeds of which 

 seem to lose their buoyancy in inland stations. Not all littoral 

 plants, however, lose the floating power of the seeds when grown 

 away from the coast. The seeds of Ipomea pes caprse retain it in 

 spite of the change of station. This point is dealt with in 

 Chapter XIII and in Note 44. 



In concluding this general sketch of the first results obtained 

 by testing the buoyancy in sea-water of a collection of seeds and 

 fruits from a mountainous Pacific island, such as we find in Fiji, I 

 must remind the reader that the subject has only been lightly 



