REPRODUCTION. 



197 



spores. Tlie 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, tlie ccqnl' 

 litlum. 



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

 single terminal spores or chains of spores. Among tlie 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). HeteroBcie 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 heteroecious 

 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 

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

 tion : in Puccinia straminis the Borraginece,.^ and in Puccinia 

 graminis i\\Q shrub Berheris mdgaris (see Fig. 123). 2. To de- 



FiG. 123. — A, Young secidium ; la, mature aecidia {a) on a leaf-section of Berberis 



vulgaris; B, highly magnified teleutospore 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 



