io8 



Minnesota Plant Diseases. 



gether with constant moisture. These fungi are close relatives- 

 of the water-molds, differing from them in the long-beaded 

 shape of their threads and in the presence of but one egg in the 

 egg-case. Swarm-spores are again the chief means of distribu- 

 tion, though bits of the threads may be carried along in the 

 pipes and become lodged. By growth these again produce an- 

 other colony. In all kinds of drain-pipes,, 

 in sewers or in the drains of refrigerators- 

 these molds abound. They form dense, 

 compact masses of mycelium which may 

 ultimately stop up the pipe and thus 

 cause trouble. They may also abound in 

 streams, below factories or at the mouths 

 of sewers, and may form large woolly 

 masses of mycelium. (Fig. 43.) 



Damping-off fungi (Saprolegniinecein 

 part). These molds are also relatives of 

 the water-molds but have approached 

 more nearly to the terrestrial habit. In 

 fact many of them are able to live com- 

 fortably in a very moist atmosphere, 

 though typically they live in or at the 

 surface of the water. The damping-off 

 fungi have swimming spores and egg- 

 spores; only one egg is produced in a 

 case and the breeding between egg and 

 male elements has been observed. The 

 egg-spore is again a resting spore. Like 

 the fish- and water-molds the damping- 

 off fungi may live as saprophytes upon 

 dead organic matter in the water. 

 Many of them are, however, parasitic on 

 algae, such as pond scums and green 

 felts, while a few attack pinworms. By 

 far the most common forms are, how- 

 ever, those which cause the disease known as damping-off. 

 When seedlings, particularly of the mustard family, are sown 

 very thick and are kept very moist, the damping-off fungi ap- 

 pear. They attack the seedlings just at the surface of the soil or 



FIG. 43 Sewer-pipe fungi. 1. 

 Fungus threads with pecu- 

 liar constrictions. 2. Same 

 (more highly magnified) , 

 with characteristic gran- 

 ules near constrictions. 3. 

 Swimming spores. High- 

 ly magnified. After 

 Pringsheim. 



