(k) In support of this contention it is pointed out that most of the Hawaiian strand-plants that are dispersed by the currents are found in America, and some indeed in America to the exclusion of the Old World.
(l) The arrangement of the currents in the North Pacific also favours the view that the Hawaiian Islands are more likely to receive plants by the agency of the currents from America than from the Asiatic side of the Pacific.
CHAPTER IX
THE GERMINATION OF FLOATING SEEDS
Germination in the floating seed-drift of tropical estuaries.—A strain of vivipary.—Abortive germination of seeds in warm seas.—A barrier to plant dispersal.—The borderland of vivipary.— Summary.
The tendency of the floating seed or fruit to germinate in the estuaries of tropical rivers is especially characteristic of the plants of the mangrove-swamps and of their borders. In the Fijian rivers, and particularly in the estuary of the Rewa, where the river-water is usually mixed with that of the sea, there are frequently to be found in a state of germination floating fruits of Barringtonia racemosa, Carapa obovata, Clerodendron inerme, Derris uliginosa, Smythea pacifica, &c.; whilst the floating fruits of more characteristic beach-trees like Barringtonia speciosa and Cerbera Odollam, that grow also on the sides of the estuaries, were never noticed in this condition. That this tendency should be restricted to the plants of the mangrove-formation and is not to be observed in the beach-trees is a singular fact. There is, however, an intermediate group of littoral plants mostly belonging to genera of the Leguminosæ and Convolvulaceæ, such as Mucuna and Ipomœa, where germination of the floating seed is apt to begin but ends abortively, and results in the sinking and death of the seed. The subject of the germination of seeds in the floating drift of tropical estuaries presents itself, therefore, in three aspects:—
(1) As concerning the plants of the mangrove-formation, where, excluding the viviparous species (when germination takes place on the plant), germination is frequent in the water:
(2) As concerning the beach-trees where it is rare or absent altogether:
(3) As concerning certain Leguminous and Convolvulaceous littoral plants where germination is not infrequent but always abortive.
Dealing first with the plants of the mangrove-formation, it may be remarked that the same tendency of the floating fruits or seeds to germinate, which is above noticed in the case of the estuaries of Fiji, came under my observation in the floating drift of the estuary of the Guayaquil River in Ecuador, the germinating fruits and seeds being carried far out to sea. The seeds of Anona paludosa, which float in quantities in the river-drift, were often found germinating; and the same may be said of the fruits of Laguncularia racemosa and of the “joints” of Salicornia peruviana which abound in the creeks of the mangrove-delta and are carried out to sea in the germinating condition.
It might be expected that this readiness to germinate in the brackish water of estuaries would prove to be a formidable obstacle to the dispersal of these plants over wide tracts of ocean. The exposed portions of the seedling might be deemed ill-suited to withstand, without injury, the “wear-and-tear” of transport by currents over long distances, even when not affected by the sea-water; and it might be thought that they would be often nibbled off by fish or destroyed by other aquatic animals. Only the specially organised seedlings produced by a viviparous process on the tree, such as those of Rhizophora and Bruguiera, might be regarded as able to survive the effects of prolonged immersion in the oceanic currents.