Clare Island Survey — Phanerogamia. 10 61 



non- buoyancy, as such, are merely fortuitous characters ; that the structure of 

 the fruits or seeds has been determined by causes other than the relation of 

 their specific gravity to that of fresh or salt water ; and that while seed- 

 buoyancy has in some cases greatly assisted dispersal, it has played no part, 

 through the agency of natural selection, in moulding the fruit or seed to its 

 present form 1 — an opinion divergent from that expressed by Schimper 2 and 

 others. However this may be, we see at once that the absence of buoyancy 

 in the seeds of the bulk of the flora renders it impossible that the present 

 vegetation of Clare Island was derived by sea-transport from the mainland, 

 across the existing water-barrier. 



Again, if we analyze the local flora according to Chippy's buoyancy tables, 

 we obtain the following result. Of the 333 British species experimented on 

 by him, 79; or 23'7 per cent., 3 were found to possess the power of floating for 

 a week or more. Out of these 333 species, 115 grow on Clare Island; and 

 of these, 37, or 24 - 3 per cent., belong to the floating section. This group of 

 37 species represents 94 per cent, of the phanerogamic flora of Clare 

 Island. For lack of information respecting the specific gravity of the seeds 

 of many other local plants, this analysis is incomplete. Nevertheless we 

 see that, so far as the information takes us, the flora of Clare Island shows 

 no significant preponderance of species which bear buoyant seeds, and which 

 might therefore have been introduced by sea. The proportion is a little 

 higher than that suggested by Darwin for floras in general, and by Guppy 

 for the British flora. 



It may be pointed out also that in the case of Clare Island, geographical 

 and meteorological conditions are against the introduction of plants by 

 water. In the first place, the seeds of land-plants reach the sea mainly by 

 the agency of rivers. The enormous number of seeds carried by a consider- 

 able stream flowing through a region with a dense plant population is 4 

 evident from Guppy's study of the Thames as an agency of plant-dispersal. 

 But as regards Clare Island, a glance at a map shows us that along the whole 

 Mayo shore the rivers are insignificant, owing to the fact that the watershed 

 lies close to the coast. Between the Moy and the Corrib no river of any 

 importance enters the ocean; and on the mainland adjoining Clare Island the 

 streams are especially small. Again, while the current setting into and out 

 of Clew Bay with each tide no doubt carries flotsam to or past the island, 

 the prevailing wind, westerly (north-west to south-west) for some 183 days 



1 H. B. Goppy, he. eit., chap. xiii. 



2 A. F. "W. Schisiper: Plant- Geography (English ed.), p. 28. 1903. 



3 The high percentage is due to the fact that seeds likely to float were especially studied. 



4 H. B. Guppy: The Itiver Thames as an Agent in Plant-Dispersal. Journ. Linn. Soc. 

 (Botany), xxix, pp. 333-346. 1893. 



