The Artificial Infection of Plants. 1 1 7 



with a dry camel-hair pencil, taking care to use only those 

 which will readily brush off. The contents of the watch- 

 glass may then be applied to the wheat plants. By this 

 method the accidental infection of the control plants is 

 avoided, because the bell-glass is never removed from 

 them. 



Personally, I have found infection with promycelial 

 spores more certain than with the aecidiospores, because 

 we can see that they are actually germinating at the time 

 they are used, while with the aecidiospores this cannot 

 be done so certainly ; moreover, the a;cidiospores being 

 generally brought from a distance, they are apt to lose 

 this germinative power, unless used immediately, from 

 becoming too dry on the one hand, or on the other, if kept 

 in too moist an atmosphere during their transit, from many 

 of them having already germinated before they are em- 

 ployed. It is only by attention to these minute details 

 that we can ensure uniform success. 



The Gymnosporangia are very easy to cultivate. A 



few seedling hawthorns can be obtained anywhere, and it 



is necessary only to soak the Gymnosporangiinn clavarics- 



forme for twelve hours, when the golden promycelial spores 



will be visible to the naked eye. 



In producing R. cancellata on pear, it is necessary to 

 infect two-year old plants, because if seedlings be infected 

 the spermogonia alone will be produced, because seedling 

 pears lose their foliage before the Roestelia has had time 

 to develop ; with these plants it is essential to success 

 that they should be thoroughly established before they are 

 made the subject of experiment- 

 In working out the life-history of the allied species 

 duplicated cultures are very valuable. Suppose we wish 

 to produce the eecidia of P. magnusiana and P. pJirngmitis. 

 Having provided the proper material and a number of 



