PARASITIC FUNGI 



173 



the point of union is dissolved, permitting the union of the 

 contents of the two cells. This results in the formation of the 

 zygospore which is capable of giving rise to a new plant. The 

 sexual method of reproduction rarely occurs in nature. 



The species of Saprolegnia (Fig. 112) are parasitic on fish, 

 but will grow on floating dead flies and other insects and occa- 

 sionally on decaying food. Thread-like filaments of the fungus 

 grow out from the material on which it is living. Cross walls 

 or septa are formed in these filaments and great numbers of 



Fio. 111. Rhizopus nigricans or bread mould; a, entire plant showing sporangia; b, 



mature zygoapore. 



unicellular, free-swimming zoospores will be produced in the 

 apical cell. These zoospores escape, swim for a time, become 

 attached to suitable food material and produce new plants. Cer- 

 tain of the filaments will also produce antheridia and ob'gonia, 

 giving rise to ob'spores, but there is some doubt as to whether 

 fertilization really occurs. 



Parasitic Fungi. The downy mildews, powdery mildews, 

 and many other fungi are parasitic on the higher plants and are 

 causes of heavy losses to our agricultural interests every year. 



