xxx RHIZOPHORA 459 



dispersal to a distant age in the distribution of shore-plants of the 

 mangrove type. 



When Schimper published his work on the Indo-Malayan 

 strand flora in 1891, but little was known of the duration of the 

 floating capacity of Rhizophora seedlings (p. 166). In giving the 

 results of my investigations I am merely describing the agencies 

 of dispersal at present in operation. Such agencies have their 

 limitations, and we may, perhaps, be thus able to explain why 

 Rhizophora is restricted in the Pacific islands to the archipelagoes 

 of the Western Pacific ; but many serious objections would at once 

 present themselves if we regarded the occurrence of the genus in 

 America, as well as in Asia and Africa, as a matter depending on 

 capacities and means of dispersal. 



The fruits of Rhizophora, as they display themselves before the 

 protrusion of the germinating seed, have no buoyancy, and the 

 germinating fruits until the hypocotyl has protruded for some 

 inches (6 inches in the case of R. mangle) also sink in sea-water. 

 With a further increase in the length of the hypocotyl, the ger- 

 minating fruit acquires buoyancy ; and when the seedling, usually 

 10 or ii inches in length, becomes detached from the fruit on the 

 tree and falls into the sea, it floats readily in 95 per cent, of the 

 cases. Such seedlings occur very commonly in the floating drift of 

 the estuaries and out at sea both in Fiji and in Ecuador. 



Out of five seedlings of the Asiatic species, Rhizophora mucro- 

 nata, that had fallen naturally from the tree, three were afloat and 

 healthy after eighty-seven days' immersion in sea- water. Out of 

 twenty seedlings of the American mangrove, Rhizophora mangle, 

 sixteen floated after ninety days and four were afloat and healthy 

 after one hundred and twenty days, the greater number sinking 

 during the fourth month. These results indicate considerable 

 powers of buoyancy, and go to show that extensive tracts of ocean 

 could be traversed by the floating seedling. 



It should, however, be observed that not all the full-sized 

 seedlings float. With Rhizophora mangle about 5 per cent, sink in 

 sea-water and from 20 to 50 per cent, sink in fresh-water ; whilst with 

 R. mucronata the proportion of non-buoyant seedlings is rather 

 greater. There would thus appear to be a rather nice adjustment 

 of the specific weight of the seedlings to the density of sea-water- 

 Generally speaking, they may be seen floating vertically or steeply 

 inclined in the fresh-water of estuaries and horizontally in the sea. 

 With the buoyant seedlings of Rhizophora mucronata, as a rule, 

 about 90 per cent, float horizontally in sea-water, and about 70 per 



