SEED-DISPERSAL 259 



roots by a gust of wind, and is trundled along the surface 

 like a light wicker ball, the ripe seed-vessels dropping their 

 seeds by the way. In case of an obstruction, such as a 

 fence, great masses of these tumble weeds may be seen 

 lodged against the windward side. 



This method of dispersal is far more effective than the 

 mechanical discharge; but it is fitful, and its range usually 

 is not very great. Thistle-down may be floated into a 

 neighboring field, and a strong wind may carry the com- 

 paratively heavy-winged fruits of the maple and the elm 

 some distance; but at best the scattering is only over a 

 neighborhood. 



153. Dispersal by currents of water. Many seeds are 

 buoyant, or become so after soaking in water, and may be 

 carried great distances by currents. For example, the 

 banks and flood-plains of streams may receive seeds from 

 a wide area, dependent on the extent of the drainage system. 

 Along the lower stretches of rivers such as the Mississippi, 

 the Missouri, or the Ohio, almost every season new plants 

 are added to those growing along the banks, and some 

 of them may have come from great distances. This kind 

 of distribution, therefore, may become almost continental 

 in extent. 



Still more far-reaching is the dispersal brought about 

 by oceanic currents, both by waves carrying seeds along 

 the coast, and also by the deeper currents that extend 

 from continent to continent or to oceanic islands. It has 

 been found that many seeds can endure even prolonged 

 soaking in sea-water and then germinate. From a series 

 of experiments, Darwin estimated that at least fourteen 

 per cent of the seeds of British plants can retain their 

 vitality in sea- water for twenty-eight days. At the 

 ordinary rate of movement of ocean currents, this length 

 of time would permit seeds to be transported over a thousand 

 miles. It is thought that the appearance on islands of 



