WIND DISPERSAL 



415 



The hairy tufts and plumes of such seeds as the cotton, 

 the silk-weeds (Asclepias sps.), the willow herbs, the willow, 

 and such fruits as those of the Compositae and cotton-grass 

 are much more efficient. Small (19 17) has shown that the 

 pappus of the Compositae enables the fruit to act as a glider 

 in very light breezes. The fruit of the dandelion is kept 

 afloat indefinitely in a wind of only 2 miles per hour, while 

 that of the colt's foot requires a wind of only 0*5 mile per 



Fig. 66. — Dispersal of fruits and seeds : i , winged seed (Bignoniaceae) ; 

 2, winged fruit of maple; 3, plumed fruit of goat's-beard ; 4, hairy- 

 fruit of cotton-grass. Nat. size. 



hour. The possibility of dispersal of such fruits is almost 

 limitless and must account for the ubiquity of such genera 

 as Senecio. Possibly twenty-eight species of the new flora 

 of Krakatau (30 per cent, of the whole) have been carried 

 there by wind. 



C. Dispersal by Water Currents is well illustrated by the 

 occurrence of alpine plants on river shingles far below their 

 proper level and many miles from their natural stations. 

 Yet dispersal by such means is quite limited in its extent 



