172 ELEMENTARY STUDIES IN BOTANY 



the plant is easily broken from its roots by a gust of wind, 

 and is trundled along the surface like a light wicker ball 

 (Fig. 141), the ripe seed-vessels dropping their seeds by the 

 way. Wind-dispersal is far more effective than mechanical 

 discharge, but it is fitful, and its range usually is not very 

 great. " Thistledown " may be floated into a neighboring 

 field, and a strong wind may carry the comparatively heavy- 

 winged fruits of the maple and elm some distance, but at 

 best the scattering is only over a neighborhood. 



FIG. 141. A common tumbleweed. 



A wide-ranging method of dispersal is by means of currents 

 of water. For example, the banks and flood-plains of streams 

 may receive seeds from a wide area, dependent upon the 

 extent of the drainage system. Along the lower stretches 

 of such rivers as the Mississippi, the Missouri, and 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 



