186 PLANTS, SEEDS, AND CURRENTS 



and Cornwall are concerned with its transient occupation of the 

 beaches in places usually within reach of the wash of the waves at 

 the higher tides. A few individuals occupy a beach for a season 

 or two, then disappear, and two or three or more years elapse before 

 the species again gets a footing. 



An interesting feature in the distribution of the American plants 

 is the occurrence of two Atlantic coast species (C. americana and 

 C. edentula) on the shores of the Great Lakes. Here in the society 

 of other beach plants of the same Atlantic beaches, such as Lathyrus 

 maritimus, Euphorbia polygonifolia, Psamma arenaria, etc., these 

 maritime species have found a home on the borders of inland fresh- 

 water lakes. This invasion of what is now the interior of a continent 

 is regarded by Harshberger (p. 222) as having taken place during 

 the post-glacial submergence of these regions. Such a change of 

 station throws some suspicion on the determining influence of the 

 halophilous inclination when it is so readily thrown off. In this 

 connection one is inclined to recall Kearney's discovery that the 

 amount of salt in sea-beaches is a negligible quantity, and to ask 

 oneself what constant character in the organisation of Cakile plants 

 we can associate with the double station on a sea-beach and on the 

 shores of a fresh- water lake. (For comment on Kearney's discovery 

 see Harshberger in Proc. Amer. Philos. Soc, 1909.) 



That the currents are active dispersers of these plants may be 

 inferred from their frequent occurrence in isolated islands, as exem- 

 plified below in the case of Cakile lanceolata, though from facts to 

 be subsequently given it may be doubted whether the most buoyant 

 fruits could safely cross a tract of sea more than 300 or 400 miles 

 in width. The difficulties likely to be encountered before a footing 

 can be obtained on an island in the open sea have already been 

 mentioned in the case of the Turks Islands. But they are also 

 illustrated in the case of the Florida sand-keys which are for the 

 most part curving sand-banks, raised three or four feet above the 

 sea. Although Mr. Lansing noted the occurrence of Cakile fusi- 

 formis on twelve of the nineteen keys examined by him, the species 

 was as a rule merely represented by a few scattered clumps, and 

 in only two or three instances was it fairly frequent. Always it 

 formed the outposts of the sand- vegetation on the weather side of 

 the key. But Cakile may experience the same difficulties on Euro- 

 pean beaches in obtaining a footing beyond the reach of the waves 

 in stormy weather. What the waves give, they can also remove, 

 was the lesson taught to me by Cakile on English beaches. 



On a long beach on the north coast of Cornwall it would be typically 

 represented by solitary clumps of C. maritima lying far apart and 

 all below the highest line of drift. They are derived from stranded 

 fruits that have germinated in the drift, of which results may be 

 seen in the numerous seedlings growing in the midst of the rubbish 

 heaped up by the waves. It is in the upcast wrack of the Norwegian 

 beaches that this species of Cakile flowers and fruits (Sernander, 

 p. 123). 



Cakile lanceolata is the West Indian species most characteristic 

 of small isolated island-groups. Thus it occurs in the Cayman 



