10 60 Proceedings of the Royal Irish Academy. 



the introducers claim that it is highly interesting to watch their progress. In 

 the great majority of cases we find that fortunately these species utterly 

 vanish after a few years." 



We may now turn to the three principal means of dispersal across a 

 sea-barrier, which have been already referred to. 



Transport by Water. 



With regard to the possibility of the introduction of the flora by sea, 

 through the medium of surface-drift or currents ; it was long since shown 

 by Darwin, 1 Martins, 2 and Thuret 3 that few seeds are able to remain long 

 afloat in water, even if they float at all ; though many are not killed by 

 the sea-water. Darwin's conclusion in this matter is that, even when dried 

 seeds and dried branches bearing seeds are taken into account, not more than 

 10 per cent, of any flora is capable of a long sea journey and subsequent 

 germination. We may turn to the observations recorded by H. B. G-uppy. 4 

 This naturalist records the results of experiments (made mostly by himself) 

 on the floating power of 333 species of British flowering plants. General- 

 izing from these results, he concludes that about 90 per cent, of British 

 Phanerogams have non-buoyant seeds — that is, they either sink at once, 

 in water either fresh or salt, or sink after immersion for a day or two. The 

 seeds are found to be usually either considerably heavier or considerably lighter 

 than water. The balance of 10 per cent., in which the seeds are buoyant, consists 

 almost entirely of river-side or marsh plants on the one hand, and sea-side plants 

 on the other, the former class largely predominating ; but buoyancy is not 

 characteristic of the seeds of the majority even of the plants of such 

 situations. 6 While without question sea-dispersal has played a prominent 

 part in plant-distribution in the Tropics, Guppy remarks that " dispersal by 

 currents seem to be mainly restricted to warm latitudes." " There is very 

 little effective dispersal by currents in temperate regions." 6 " Ocean currents," 

 says Blytt, 7 speaking of northern latitudes, " without drifting ice, are 

 certainly no powerful means of transport." Guppy argues that buoyancy and 



1 Darwin : Origin of Species, 6th ed., pp. 506-508. 



2 Ch. Martins : Experiences sur la persistence de la vitalite des graines fiottant a la surface de 

 la mer. Bull. Soc. Bot. de France, iv., 1857, pp. 324-337. 



3 Gustave Thukzt : Experiences sur les graines des diverses especes plongees dans l'eau de 

 mer. Bibl. Univ. et Bevue Suisse, Geneve, N.P., xlvii., 1873, pp. 177-194. 



1 H. B. Gdppy: Observations of a Naturalist in the Pacific between 1896 and 1899. Vol. ii., 

 Plant-Dispersal. Note 10, pp. 535 538. 1906. 

 6 Ibid., pp. 24, 25. 



6 Ibid., pp. 429, 432. 



7 A. Bia'ti : loin, tit., p. 30. 



