THE DISPERSAL OF SPORES BY THE WIND 561 



speed of the wind only 1 mile per hour, the spores before settling 

 would still travel 14 miles. ^ 



In the above theoretical case, the assumptions made were of 

 the simplest kind, so that the problem of the carriage of the spores 

 could be easily solved ; but, in nature, the infected Ribes bushes 

 are often situated amid low-lying forests, and the currents of air 

 blowing over the surface of land covered by vegetation take the 

 most various courses owing to the unevenness of the land surface, 

 the irregular disposition of trees, shrubs, and herbs, local convection 

 currents, and the change of winds. Truly, unless one is prepared 

 to make an almost infinite series of observations : " The wind 

 bloweth where it listeth, and thou . . . canst not tell whence it 

 Cometh and whither it goeth." Moreover, if to the uncertainty of 

 our knowledge concerning the direction and speed of air-currents 

 in forests, etc., be added our even yet considerable ignorance of the 

 conditions of spore germination and infection in nature, the distri- 

 bution of the infected plants of the two hosts of a heteroecious 

 rust-fungus, such as Ribes smdPinus Strobusior Cronartium ribicola, 

 should be for the present — as it actually is — often not a little 

 perplexing to field observers. 



Uredospores differ from basidiospores and aecidiospores in that 

 they are not violently shot away from the places where they are 

 developed. They accumulate at the surface of uredospore-sori, and 

 it is not until the wind is blowing with a certain speed that they 

 are carried away. Thus their dispersal is initiated like that of the 

 spores of Puff-balls and of Mycetozoa. Uredospores, of course, are 

 carried about by air-currents in the same manner as basidiospores ; 



^ According to Perley Spaulding (" Investigations of the White-Pine BUster 

 Rust," U.S. Department of Agriculture, Bull. No. 957, 1922, pp. 67-68), the experi- 

 ments of York, Overholts, and Taylor indicate that the basidiospores of C. ribicola 

 are very short-lived and, after their liberation, can scarcely survive drying for a 

 period of 10 minutes. Abundant moisture is evidently necessary for infection of 

 pines. It seems not unUkely that in nature, under moist conditions, the basidio- 

 spores, after being shot from their sterigmata, retain their vitality and power of 

 germination for a much longer time than 10 minutes. However, the problem 

 discussed above is one of the distance to which the basidiospores may be carried 

 by the wind, not one of spore-germination. It is possible that the basidiospores, 

 when carried long distances by the wind, cannot infect pines upon which they 

 settle owing to loss of vitality in transit. 



VOL. m. 2 



