204 A RELATIVE OF THE BLACK MOULDS 



once and produce a new plant under suitable conditions of mois- 

 ture and food, or they will retain their vitality for months if kept 

 dry. As the spores are disseminated, the dome-like structure"^ in 

 the sporangium reflexes, owing to the loss of its watery contents 

 and assumes an umbrella shape, as seen in Fig. 135, F. You 

 can readily understand why these black moulds are so common 

 by counting the number of sporangia on a small bit of mycelium 

 and then estimating the number of spores in a sporangium. So 

 numerous and light are the spores that they are carried every- 

 where. It is only necessary to expose a bit of moist bread for a 

 few moments to the air and then enclose it in a damp chamber to 

 secure a luxuriant crop of these plants. Under certain conditions 

 the sexual method of reproduction is effected by the union of 

 two club-shaped hyphae, as shown in Fig. 136, A. As these 

 hyphae meet, the tip of each branch is cut off by a wall and the 



FIG. 137. A fungus, Empusa, parasitic upon flies : A, fly surrounded by 

 a mass of discharged sporangia. B, enlarged view of several hyphae, 

 showing the discharge of the sporangia which are surrounded by a mu- 

 cilaginous substance. After Brefeld. 



