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. 251). Plumes 
and tufts of hair are devel- 
Fic. 252.—Akenes of dandelion with Fic. 253.—Akenes of Senecio with tufts 
tufts of hair—After Kerner. 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), ete. 
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 
