GEOGRAPHICAL DISTRIBUTION 147 



species on widely separated mountain tops in the Malay 

 Archipelago where wind (particularly the west monsoon) 

 is the only agent of dispersal that may reasonably be as- 

 signed. The seeds of many plants are as light as dust 

 particles, and it has been calculated that nearly 850,000,- 

 000 tons of dust are transported as far as 1,440 miles a 

 year in the western United States.^ In the light of this 

 information it is not difficult to understand how seeds of 

 Nepenthes phyllamorpha, that weight only 0.000035 gram, 

 seeds of Rhododendron verticillatum and of Dendrohiuni 

 attenuatum, that weigh 0.000028 gram and 0.00000565 

 gram respectively, can be transported many miles, re- 

 sulting in a geographical distribution of those (and various 

 other) species, on the mountain tops of oceanic islands that 

 are miles apart. 



James Small has carried out a series of painstaking 

 experiments on the transportation of the seeds of various 

 plants by artificially produced air currents. Among many 

 valuable results of these experiments, he determined that 

 for the seeds of the dandelion, "so long as the relative hu- 

 midity of the air remains above 0.77 per cent, and so long 

 as the fruit does not encounter an obstacle, a horizontal 

 wind of 1.97 miles per hour is sufficient for its dispersal to 

 any distance. If the air becomes moist the pappus closes up 

 and the fruit falls rapidly." Small further concludes that 

 the ordinary pappose fruit of the Compositae, under the 

 proper meteorological conditions, can be blown many hun- 

 dreds of miles over land and sea, and "that hypothetical 

 land bridges are not necessary to explain the present dis- 

 tribution of the Compositae, so that we can take the world 



' Cited byjames Small (iV^ewPAyiofogj.si 17: 226. 1918) from Evans, J. W. 

 The wearing down of rocks, Pt. II, Proc.Geol. Assoc. 25, 'Pt.i^:22g. 1914. 



