PARASITIC FUNGI AND MOULDS. 29 
very small, and are formed within a small enlargement 
(sporangiwm) 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). 
