322 THE RUSTS OF NOVA SCOTIA. FRASER. 



germinate unless exposed to the weather for some time. If 

 kept inside during the winter germination does not follow even 

 if placed under favourable conditions. 



After exposure to the weather for some time, they usually 

 germinate under favourable conditions, though germination is 

 somewhat uncertain. A tube emerges from the germ pore and 

 soon divides into four, each segment is called a basidium and 

 produces on a sterigma a small, usually hyaline spore. These 

 spores, called basidiospores (sporidia), when they reach a suit- 

 able host germinate, the germ tubes make their way through 

 the epidermis and infection results. 



When the teliospores germinate at once usually the other 

 spore forms are suppressed. They are not lost, however, as 

 they occasionally make their appearance. When the telio- 

 spores germinate at once the germ-tube of the b0 Q icliospore is 

 said to enter at the breathing pore. 



Mesospores. 



In many rusts of the genus Puccinia, beside the usual two- 

 ctellledi spores large numbers of the single-celled spores are 

 formed. These resemble the teliospores and function like 

 them, the only difference being in form and the number of 

 cells. They are called mesospores. They are common, espee 1 

 ially in some species of the grass and sedge rusts. The presence 

 of these one-celled spores seems to indicate a close relationship 

 to the genus Uromyces. It is possible that the genus Puccinia 

 developed from Uromyces. 



Pleteroecism. 



Many rusts pass part of their life cycle on one plant and 

 part on another, while in some species the whole life cycle is 

 confined to one host plant. Those belonging to the former are 

 called heteroecious species and the latter autoecious species. 

 In heteroecious species one host plant bears tho pycnia and 

 aecia, the other the uredinia and telia. Usually the host plants 



