548 RESEARCHES ON FUNGI 



in such a way as to cause the several basidia to arise in situations 

 which will lead to least mutual basidial interference ; and to attain 

 this end several exits for the basidium of each cell would be of 

 advantage. However, exact observations only can decide the value 

 of this suggestion and solve the problem which has been raised. 



The Number of Uredospores and Aecidiospores of Puccinia 

 graminis, — The number of aecidiospores, uredospores, teleutospores, 

 and basidiospores produced by different species of Rust fungi varies 

 greatly, more especially in accordance with the amount of infection 

 of the host-plant. The following remarks will be confined to 

 Puccinia graminis. 



The number of uredospores of Puccinia graminis produced upon 

 a single badly infected wheat-plant amounts to many millions ; 

 and it is probable that the number of spores which come into 

 existence upon the wheat of a single farm in Western Canada would 

 often be far more than sufficient to infect all the wheat-plants in 

 the world, could they be spread upon them and germinate under 

 favourable conditions. When once uredospores have come to be 

 produced in a wheat area, the extraordinarily rapid spread of the 

 Black Stem Rust Disease in moist weather is exactly what one would 

 expect to happen if due regard be given to the high geometrical 

 increase in the number of the organs of dissemination in successive 

 generations. In Volume I of this work, I described a puff-ball of 

 Lycojperdon giganteum {cf. Fig. 224) which produced 7,000,000,000,000 

 spores. I have since calculated that, if every spore of this puff-ball 

 had germinated and given rise to a puff-ball like its parent, and if 

 every spore of the second-generation puff-balls had likewise ger- 

 minated and given rise to a puff-ball like its parent, then, at the 

 end of these two filial generations only, there would have come into 

 existence a mass of puff-ball matter equal to 800 globes the size of 

 the planet on which we live ! From a single uredospore which has 

 infected the leaf or stem of a wheat-plant, several thousands of 

 new uredospores may be produced in the course of seven or eight 

 days, so that successive generations of uredospores can be produced 

 under favourable conditions with great rapidity. On the other 

 hand, in a Puff-ball, successive generations of spores are probably 

 not produced in less than a year. After all, therefore, under ideal 



