THE DISPERSAL OF SPORES BY THE WIND 565 



relatively numerous up to 11,000 feet, but were much scarcer at 



greater heights. Two spores of what appeared to be Puccinia 



triticina (the cause of the Leaf Rust Disease of wheat) were caught at 



a height of 1 6,500 feet or, roughly, 3 miles above the ground. Uredo- 



spores and aecidiospores of Puccinia graminis caught at elevations of 



7,000 feet and 1,000 feet, respectively, were found able to germinate. 



Further evidence of long-distance transportation by the wind is 



provided by the observations of L. H. Pennington 1 on the spread 



of Cronartium ribicola from pines to currant bushes. Recently 



Pennington observed that, in the Pacific North-West of North 



America (which region has lately been invaded from the east by G. 



ribicola), certain currant bushes have become infected by the fungus 



and bear uredo-sori, although they are situated 110 miles beyond 



the limit of the geographical distribution of the White Pine {Pinus 



Strobus) from which the infecting aecidiospores must have come. 



Not only spores of fungi but any other microscopic particles 

 must, on account of their slow terminal rate of fall in still air, often 

 be carried long distances by the wind before they come to earth. 

 This has been shown to be the case for Lycopodium powder and 

 pollen grains by Schmidt and Hesselman. Schmidts shows that 

 " Lycopodium spores which have a rate of fall of 1 • 76 cm. a second, 

 when liberated in a wind blowing 36 kilometres (22-5 miles) an 

 hour possess an average limit of distribution of 330 kilometres 

 (206 • 25 miles). Similarly, even in the case of Pinus sylvestris pollen, 

 which has as great a diameter as 48 fi, and as rapid a fall as 5 • 3 cm.' 

 a second, he computes that as many as four-tenths of the number 

 originally shed would be carried 4 kilometres by a wind of 36 kilo- 

 metres an hour ; while two-tenths would reach 13 kilometres ; and 

 one-hundredth would reach 36 kilometres. The work of Hessel- 

 man ^ shows that these calculations represent the conditions 

 attained in nature. He found that conifer pollen grains 30 to 60 fi 



■J L. K Pennington, "Wind Dissemination of Aeciospores of Cronartium 

 ribicola, Phytopathology, vol. xiv, 1924, pp. 52-53. 



2 w. Schmidt, " Die Verbreitung von Samen und Blutenstaub durch Luft- 

 bewegung, Osterr. Bot. Zeitschrift, Jahrg. LXVII, 1918, pp. 313-328. 

 /T, ^\ Hesselman, " lakttagelser over skogstradspollens spridninsformaga 

 (Beobachtungen uber die Verbreitungsfahigkeit des Waldbaumpollens). " In Meddel 

 ktratens Skogsjorsoksanst (Sweden), Haft XVI, 1919, pp 27-60 



