80 THE FACTORS [Part I 



carried inland, where halophytes could not establish themselves, or into 

 the sea, where only floating devices could prevent them from sinking. 

 Weaker breezes which suffice for the transport of pollen are less efficacious 

 for seeds, and all the more so because the smooth loose sand does not 

 arrest anemophilous seeds that have fallen to the ground, but leaves them 

 to the play of the wind. 



The work of the wind as a means of seed-dispersal and of spore-dispersal 

 is one of the most important subjects in geographical botany. We cannot 

 say that a conclusive opinion has yet been arrived at regarding it. A. de 

 Candolle and Kerner estimate the efficiency of the wind in this respect at 

 a very low figure in the case of seed-plants. Seeds, they maintain, are 

 conveyed by the wind to short distances only. The former botanist, how- 

 ever, admits the possibility of a longer transport for the spores of crypto- 

 gamous plants. According to this view the dispersal of seeds by the wind 

 is merely a local phenomenon and would acquire geographical importance 

 only when frequently repeated in the course of generations. This view is 

 supported by the fact that the transport by the wind of seeds and spores 

 over extensive tracts of water, to oceanic islands for instance, has not yet, 

 in spite of repeated assertions, been positively proved. On the other hand, 

 the presence of various species of plants on such islands can be explained 

 only on the hypothesis of the intervention of the wind. 



Treub proved that seeds can be carried by wind over stretches of the sea 

 at least twenty nautical miles in width, for he found in the interior of the 

 island of Krakatoa, which is that distance from Java, three years after the 

 eruption which had covered the island with a thick sheet of lava, eleven 

 ferns, two species of Compositae, and two grasses whose spores or seeds 

 could have been carried thither by means of the wind alone. Accordingly, 

 it is in the first place ferns from the neighbouring islands that colonized the 

 devastated interior of Krakatoa. Ferns also form the chief vegetation of 

 recent volcanic islands that are remote from continents ; for instance, the 

 little island of Ascension is almost completely overgrown with ferns. 

 Plants that are disseminated by marine currents are not as a rule provided 

 with special means for making their way inland, especially when the 

 interior of the country is mountainous ; and berry-eating birds that take 

 long flights do not, excepting for rare accidents, visit islands before trees 

 are established. Only two phanerogamic littoral plants were found inland 

 by Treub upon Krakatoa, Scaevola Koenigii and Tournefortia argentea, the 

 seeds of which are so small and light that the wind might have blown 

 them on to the mountains. Plants disseminated by animals were com- 

 pletely absent. 



The significance of anemophilous means of dispersal in relation to the 

 origin of an insular flora has been finally determined by Treub's important 

 observations. 



