56o RESEARCHES ON FUNGI 



enormous surface area, and its strongly curved exterior, it must 

 dry up, after liberation into air not completely saturated with 

 moisture, within a few seconds. 



The rate of fall of uredinous basidiospores in still air, which is 

 of considerable interest in connection with a number of phyto- 

 pathological problems, has not yet been directly measured ; but, 

 from our knowledge of the size of the spores and of the rates of fall 

 of hymenomycetous basidiospores of different sizes as given in the 

 first Volume of this work,i there can be but little doubt that it is 

 only about 1-3 mm. per second, the exact speed depending upon 

 the spore's state of desiccation. When the wind is blowing at the 

 rate of a few miles an hour, there is no theoretical reason why, 

 considering the frequent reflection of the wind from obstacles at 

 the surface of the earth and the irregularity of convection currents, 

 basidiospores should not often be carried many miles before settling. 

 Let us suppose, for instance, (1) that some bushes of Ribes infected 

 with Cronartium rihicola are situated in a valley on a hill rising 

 2,000 feet above a surrounding plain, (2) that the wind blowing 

 at the rate of 10 miles an hour sweeps up the valley and carries a 

 stream of basidiospores from the Ribes bushes over the top of the 

 hill and across the plain, (3) that the basidiospores in still air fall 

 at the rate of 3 mm. per second, and (4) that the wind current 

 carrying the spores from the top of the hill progresses in a direction 

 parallel to the plain over which it passes. Accepting these assump- 

 tions, it is clear that the basidiospores would need to fall by their 

 own weight through 2,000 feet of air before reaching the earth. 

 Now since the rate of fall is 3 mm. per second and the distance of 

 fall is 2,000 feet which equals 600,000 mm., the time taken for the 

 spores to descend to the earth whilst being borne from the top of 

 the hill across the plain would be 200,000 seconds or 56 hours ; 

 and, since the wind blows at the rate of 10 miles per hour, the 

 spores, during these 56 hours and before settling, would be borne 

 away from the hill a distance of 560 miles. If the hill were only 

 1,000 feet high instead of 2,000 and the speed of the wind only 

 5 miles an hour instead of 10, the spores before settling would be 

 carried 140 miles ; while, if the hill were only 500 feet high and the 



1 These Researches, vol. i, 1909, p. 175. 



