160 Minnesota Plant Diseases. 



complex. We will start with the winter spores. These spores 

 are in a great majority of cases resting spores and, as in the case 

 of the smut spore, are provided with very thick outer coats. 

 These winter spores may be formed singly on stalks on the ends 

 of short threads, where they are usually produced in dense clus- 

 ters, just under the host plant epidermis and are liberated as a 

 brownish or blackish powder by the rupture of this epidermis. 

 This procedure is common in grass-inhabiting rusts, in the rusts 

 of sunflower and mints. In the rusts of willow and poplar the 

 winter spores occur in a crust-forming mass, just under the 

 host cuticle, and are never shed but germinate in place. This is 

 also the case with the common golden-rod rust. The winter 

 spores in the cedar apple disease of cedars are borne in various- 

 ly-shaped masses of gelatine which expand much on absorption 

 of water and in which the winter spores germinate producing 

 the basidium-spores at the surface of the gelatine. Some, 

 again, as in the milkweed rust, produce long, thread-like bodies 

 composed entirely of winter spores. In the rust of the cow- 

 berry the winter spores remain in the cells of the host epidermis 

 and germinate there. Whatever the location or method of dis- 

 tribution of the winter spore may be, it always germinates in 

 essentially the same way. There are usually thin places in the 

 outer walls and through one of these the inner spore wall is pro- 

 truded in the form of a thread. This thread increases in length 

 as does that of the smut spores and also becomes divided, usual- 

 ly by three walls. Each of the resulting four cells sends out a 

 stalk on the end of which is formed a spore. The thread bear- 

 ing the four stalks and spores is the basidium and is noticeably 

 more definite than the smut basidium in the production of but 

 four spores, which are formed on stalks. The basidium-spores 

 are scattered by the wind, and germinate as soon as placed un- 

 der favorable conditions; they are capable of infecting host 

 plants just as is the basidium-spore of the smut. The winter- 

 spore is not the only spore-form produced by rusts. In the 

 spring the mycelium, which develops from the basidium-spore, 

 produces what is known as cluster cups. These are tiny 

 cups scarcely as large as a pin head, usually yellowish or whitish 

 in color and found in clusters. They are most commonly found 

 on conspicuous yellow spots in the host plants caused by the 



