318 THE RUSTS OF NOVA SCOTIA. ERASER. 



epidermis, the peridium breaks open, sometimes in a character- 

 istic way, and the spores escape. They are most effectively 

 distributed by the wind. 



Sometimes all unconnected aecia are placed in the form- 

 genus Aecidium, but other form-genera are used, based on 

 the presence or absence or form of the peridium. Thus, when 

 the peridium is elongated and dehisces by longitudinal slits, 

 the aecia are assigned to the form-genus Roestelia; when it is 

 extended and ruptures irregularly, to Peridermium; when 

 absent, to Caeoma; the other forms in which the peridium is 

 usually cupshaped or cylindrical, to the form-genus Aecidium. 



Aeciospores are provided with germ-pores, but they are 

 usually visible only at germination. They germinate readily 

 in water in a few hours. A germ tube is pushed out from one 

 germ pore and the contents of the cell soon pass into the tube. 

 On the host plant the tube enters through a breathing pore. 



Botanists differ as to the length of time that aeciospores 

 retain their germinating power. Plowright found they retained 

 it only a few hours, Klebahn found they retained it for weeks, 

 others found them uncertain. 



The aecia that develop from the mycelium resulting from 

 basidiospore infection are called "primary" aecia. When the 

 complete series of spore-forms is present no further aecia are 

 formed, but if the uredinia are wanting the aecia may repeat 

 themselves for some time. That is, aeciospore infection may 

 take place and produce aecia. These are called "secondary" 

 aecia. 



Urediniospores. 



The urediniospores are produced in spore masses or 

 cushions called uredinia, which usually rupture the epidermis 

 and expose the spores as a dusty mass. They may develop 

 from mycelia produced from the entrance of the germ tube of 

 a basidiospore, an aeciospore or another urediniospore. 

 Mycelial hyphae become crowded together in the host plant 



