308 PLANTS, SEEDS, AND CURRENTS 



transport, it is doubtful whether either the seeds of Convolvulus 

 soldanella or those of Sophora tetraptera would be able to withstand 

 the three-and-a-half years' immersion in sea-water that would be 

 involved in the passage. Effective dispersal of temperate shore 

 plants by the currents in the Southern Ocean could only be brought 

 about by the easterly drift in the latitudes of the Roaring Forties. 

 Very instructive is the caution given to mariners in the Admiralty 

 chart of the west coast of South America (No. 786). It warns the 

 navigator against an easterly current setting directly on the coast 

 between the latitudes of 37° and 50° S. At the southern limit, as 

 we have seen, would be stranded drift from the seas around Kerguelen 

 and from the islands south of New Zealand. Towards the northern 

 imit would be found drift from the southern part of New Zealand. 



(B) As illustrated in Tropical Latitudes. — As I have observed in my 

 previous work, dispersal by currents in tropical seas is a far more 

 effective agency than in temperate regions. The westerly set of 

 the equatorial currents is responsible for much that is held in common 

 among the rank and luxuriant vegetation of coasts and estuaries 

 in warm latitudes. The principle of looking to the east has an 

 important application when we are concerned with the existence 

 of practically the same littoral and estuarine flora on the opposite 

 sides of the same ocean, as in the cases of the Atlantic and Indian 

 Oceans. Thus, in the tropical Atlantic we should speak, not of 

 American plants in West Africa, but of West African plants in 

 America. So again in the Indian Ocean it is tropical East Africa that 

 derives its littoral flora from Malaya and North-west Australia. 

 It would be incorrect to speak of East African plants in Malaya. 

 The position adopted by Mr. Wood-Jones as regards the dispersal 

 by currents of tropical plants in the Indian Ocean has been already 

 stated. His bottle-drift experiments led him to look for Malayan 

 and Australian intruders in the African flora, or, in other words, 

 to look to the east and not to the west in order to explain the influence 

 of the currents on distribution. 



In the case of the Pacific it would be useless, for reasons before 

 given, to seek for tropical Australian plants on the west coast of 

 equatorial South America. Whatever similarities exist in the 

 littoral floras of these two regions would result from the transport 

 of seeds of South American plants westward across the ocean in the 

 South Equatorial Current. On account, however, of the great 

 breadth of the Pacific Ocean and the numerous perils to which the 

 floating seed would be exposed in an ocean traverse of from 7000 

 to 8000 miles, the Asiatic littoral plants hold the field in tropical 

 Australia. If we exclude the cosmopolitan beach and estuarine 

 plants, such as Ipomoea pes-caprce and Entada scandens, that occur 

 round the tropical zone, it is only occasionally, as with the American 

 species of Rhizophora (Rh. mangle) in Fiji, that we can find direct 

 evidence of the extension westward across the Pacific of the estuarine 

 and coast plants of the west side of equatorial South America. 

 Island groups in the track of the currents, that would be adapted 

 to serve as stepping-stones for the westward extension of the South 

 American mangrove flora, are few. 



