172 PARASITIC FUNGI 



of the mycelium are put out into the water, and each 

 is cut off by a cross wall, forming a sporangium. The 

 protoplasm of the sporangium then divides to form a 

 number of spores. These are motile spores called 

 zoospores,^ oval in shape and with flagella attached to 

 the pointed end. They are set free by the bursting of 

 the sporangium wall, swim about in the water, and 

 germinate, producing a new mycelium if they find a 

 suitable organic substratum, living or dead. Sexual 

 organs are also formed on the mycelium. These will 

 not be described here, but it may be noted that^ as 

 is often 'the case in fungi which form sexual organs, 

 conjugation may not take place, the female cell (egg) 

 developing into a new plant without fertilisation. 

 This behaviour is called parthenogenesis. 



Saprolegnia sometimes attacks fish, particularly carp 

 and gold-fish in stagnant ponds, entering by the gills, 

 which it often blocks up, causing the death of the 

 fish. According to some authorities it is a species of 

 Saprolegnia which causes salmon disease, attacking 

 the gills of the salmon, and sometimes killing them in 

 great numbers, thus causing extensive damage to 

 salmon fisheries. Other authorities, however, believe 

 that bacteria are the primary cause of this disease in the 

 salmon, which are attacked by Saprolegnia only when 

 the fish are already weakened by the bacterial attack. 



Fungal Parasites of Plants. — Fungi parasitic on the 

 higher plants are far more numerous in species and far 

 commoner than those which attack animals. They often 

 cause widespread and serious damage to crop plants. 



Pythium de Baryanum (causing the " damping off " 

 of seedlings). — Closely sown young seedlings in a 

 moist atmosphere, such as that of a greenhouse, some- 



' Greek f^ov, animal, from the motility of the spores. 



