84 A NATURALIST IN THE PACIFIC chap. 



procured the germination of a seed of Ipomea grandiflora, Lam., 

 after a year's flotation in sea-water in London, which included a 

 period of three weeks when the water temperature was at or about 

 32° F. These seeds from this point of view would be exposed to 

 much more risk of sinking through abortive attempts at germina- 

 tion when drifting across some parts of the Pacific Ocean. It 

 would appear from the Admiralty Chart of Surface-Temperatures, 

 published in 1884, that such an area with a surface-temperature of 

 83° to 86° throughout the year extends north and east of New 

 Guinea well into the Pacific, reaching in the first half of the year as 

 far east as the Tahitian region. It would seem highly probable 

 that the immersion of Leguminous or Convolvulaceous seeds for 

 many months in these tepid waters would in most if not in all 

 cases induce incipient germination which would lead to the sinking 

 of the seed. There are, however, exceptional cases, as that of 

 Caesalpinia bonducella, which, as my experiments recorded in 

 Chapter XVII. indicate, appear to be quite proof against any 

 conditions of temperature such as are likely to be found in tropical 

 seas in the present day. 



There are a few general considerations arising out of the fore- 

 going observations to which reference may now be made. The 

 study of the behaviour of the floating seed or fruit often carries us, 

 as I have before implied, to the borderland of vivipary. When 

 from a canoe on a Fijian river we lift up the germinating fruit of 

 Barringtonia racemosa from amongst the drift floating past in the 

 stream and pull down from the branches overhead the seedling a 

 foot in length of Rhizophora, we hold in our hands the two extremes 

 of the series of vivipary. With many of the plants of the mangrove- 

 formation there is a fine adjustment with respect to the germinat- 

 ing capacity of the seed, or in other words a delicate balancing of 

 organisation on one side and of physical conditions on the other. 

 A slight disturbance of the equilibrium would produce great results 

 in plant distribution. Thus, an elevation of the temperature of the 

 sea-water in the tropics to 90° F. would, I apprehend, produce the 

 abortive germination of nearly every floating seed and fruit in 

 equatorial seas, even of those of the beach-trees like Barring- 

 tonia speciosa and Terminalia littoralis that are regarded as proof 

 against such risks under existing conditions where the surface- 

 temperatures would average 78° to 80°. 



There would thus be a barrier to the dispersal of plants by 

 currents as effective as that of a frozen ocean. In the warm, humid 

 climates of the early geological ages, seed- transport by currents 



