REPRODUCTION. 



197 



spores. The spores are set free by the rupturing of the peridium, 

 while the cells of the trama enter into dissolution, except certain 

 colls which form a loose network of hyphal filaments, the capil- 

 litium. 



The Ustilaginece (blights) and Uredinece (rusts) form either 

 single terminal spores or chains of spores. Among the UredinecB 

 occurs a peculiar phenomenon called ' ' heteroecie ' ' (change of host) 

 by its discoverer, DE BARY. Successive generations live upon 

 different substrata, in this case upon different living plants (para- 

 sitic). Heteroecie is known in about fifty species of rusts. The 

 names ' ' blight ' ' and ' ' rust ' ' already indicate that we are con- 

 cerned with plant-diseases. We will first discuss the hetercecious 

 rust-fungi, then the blight-fungi. 



Puccinia graminis, the rust of our grasses, especially grains, 

 is far more injurious than the blight-disease. Blight is limited to 

 single plants of our cereals, while rust appears epidemically by its 

 rapidly formed and germinating summer-spores. The methods of 

 exterminating this plant-disease are as follows : 1. To destroy the 

 4 ' intermediate ' ' host, which serves as a substratum for one genera- 

 tion : in Puccinia straminis the jBorraginece, and in Puccinia 

 graminis the shrub Berberis vulgaris (see Fig. 123). 2. To de- 



FIG. 123. A, Young aecidium ; /, mature aecidia (a) on a lenf-sectiou of Berberis 

 vulgaris ; B, highly magnified telentospore of Puccinia graminis. 



(After Sachs.) 



stroy as many as possible of those plants which shelter the teleuto- 

 spores during the winter months, that is, the remnants left in the 



