THE DISPERSAL OF SPORES BY THE WIND 563 



of uredospores are picked up by the winds in some region in the 

 United States where Puccinia graminis is flourishing, that they are 

 dropped as a fine powder over many square miles of wheat-growing 

 territory in Manitoba and Saskatchewan, that thereby the growing 



Fig. 227. — Dispersal of aecidiospores. The young White 

 Pine, PinusStrobus, in the fore-ground bears numerous 

 aecidia of the White Pine Bhster Rust, Cronartium 

 ribicoki. The tree was struck smartly with a stick, 

 about 3 feet above the blisters. As a result vast 

 numbers of aecidiospores were set free. The spore- 

 cloud thus formed can be seen to the left of the tree. 

 Photographed at Wanensburg, New York State, by 

 H. T. Giissow. 



wheat-plants come to have uredospores lodged upon their stems 

 and leaves, and that subsequently, with the advent of very moist 

 weather, these spores germinate and infect large numbers of plants 

 simultaneously. A new crop of uredospores would then be provided 

 which, in their turn, might be caught up by the wind and infect 

 new wheat-growing areas elsewhere. 



The enormous transporting power of high winds, so far as fine 

 dust is concerned, is easy to establish theoretically, although we 

 still know but little about it from direct observations. During 



