SEED-DISPERSAL 



257 



This method may be regarded as the poorest of all the 

 methods of dispersal, for at the very best no seed-vessel 

 can discharge its seeds more than a very short distance. 



152. Dispersal by currents of air. Many seeds are so 

 light as to be carried about by currents of air. Ordinarily, 

 however, the wind-dispersed seeds or fruits develop special 

 appendages to aid in their flight, commonest among which 



are wings and tufts of hair. 

 For example, wings are de- 

 veloped by the fruit of ma- 

 ples (Fig. 250) and elms, and 

 by the seeds of catalpa and 

 its allies (Fig. 25 1 ). Plumes 

 and tufts of hair are devel- 



FIG. 252. Akenes of dandelion with 

 tufts of hair. After KERNEK. 



FIG. 253. Akenes of Senecio with tufts 

 of hair. After KERNER. 



oped by the seed-like fruits of thistle, dandelion (Fig. 

 252), and many of their relatives (Fig. 253); and by the 

 seeds of milkweeds (Fig. 254), willow herbs (Fig. 255), etc. 

 On plains, or level stretches, where winds are strong, a 

 curious habit of seed-dispersal has been developed by 

 certain plants known as tumbleweeds or field rollers (Fig. 

 256). These plants are profusely branching annuals with 



