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 tumbleweeds 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 
