PARASITIC FUNGI AND MOULDS. 



29 



very small, and are formed within a small enlargement 

 (sporangium) borne on a free hypha of the mycelium. 

 Their succession is constant and numerous as long as 

 the plant is in a favourable medium in which it can 

 flourish. The spores which are found in the same 

 medium germinate, and reproduce a mycelium similar 

 to that from which they had their origin. 



Fig. 16. Reproductive organs of Peronospora calotheca (much magnified). 



The oospores may be as much as a thousand times 

 larger in volume than ordinary spores. They are only 

 formed when the growth of the fungus is on the wane, 

 as, for instance, when the substance serving as a sup- 

 port to the mycelium is drying off: a long period may 

 elapse before they germinate (Figs. 15 and 16). 



