566 RESEARCHES ON FUNGI 



in diameter were deposited in relatively vast numbers in dishes on 

 lightships 30 kilometres (18-6 miles) and 55 kilometres (34 miles) 

 from land ; while pollen of grasses also, in spite of being liberated 

 in far less abundance, nearer the earth, and unequipped with 

 appendages for buoyancy, was caught in considerable quantities."^ 

 McCubbin ^ has measured approximately the rate of fall of the 

 uredospores of Cronartium ribicola, the White Pine Blister-Rust 

 Fungus, in still air. He released air-dried uredospores* instan- 

 taneously and automatically at the upper end of a long cardboard 

 tube 9 inches in diameter, and allowed them to fall down the tube 

 a distance of 8 feet on to glass slides filmed with glycerine. The 

 slides were set on a flat disk which revolved slowly and uniformly 

 just below the base of the tube. A small projecting pin on the 

 revolving disk tripped the release apparatus at the beginning of a 

 revolution and, as the slides passed successively under the tube, 

 they caught the falling spores. The glycerine film was then scraped 

 carefully into a line in the centre of the slide, the spores counted, 

 and their number recorded in graph form. Some of the spores took 

 five minutes to fall the 8 feet, i.e. they fell with a steady terminal 

 velocity of 8 mm. per second. McCubbin remarks : "If such spores 

 take approximately five minutes to fall a distance of 8 feet, one 

 may get some idea of their transport distance by noting that in 

 five minutes a 30-mile breeze would have carried them 2-5 miles. 

 In other words, if these spores were set free 8 feet above the surface 

 of a level plain, a 30-mile breeze would carry them 2 • 5 miles before 

 they could sink to earth or, if liberated from a small hill 32 feet 

 above the plain, they would be carried 10 miles by the same breeze 

 before being deposited." McCubbin rightly concludes from his 

 experiments that under windy weather-conditions " the radius of 

 dispersal of the urediniospores of Cronartium ribicola is to be con- 

 sidered in terms of miles rather than of yards." Doubtless this 

 conclusion applies to the uredospores of the Uredineae in general. 



1 W. H. Weston, Jr., "Production and Dispersal of Conidia in the Philippine 

 Sclerosporas of Maise," Journ. of Agric. Research, vol. xxiii, 1923, pp. 263-264. The 

 papers of Schmidt and Hesselman, at the time of writing, are inaccessible to me. 



2 W. A. McCubbin, " Dispersal distance of urediniospores of Cronartium ribi- 

 cola as indicated by their rate of fall in still air," Phytopathology, vol. viii, 1918, 

 pp. 35-36. 



