THE FUNGI 239 



themselves closely to the ascus, probably supplying it 

 with food. The outer cells of the envelope become thick- 

 walled, and form a dense protective layer, completely 

 enclosing the ascus. The envelope bears the name of 

 the perithecium ; some of its superficial cells grow out 

 into long hairs (see Fig. 97, D). The ripe perithecia 

 are visible to the naked eye, as little black dots on the 

 surface of the diseased leaf. 



The dense perithecium serves to protect the ascus 

 during the winter, for the fruits remain inactive on the 

 dead leaves until the following spring. When germina- 

 tion takes place, the ascus absorbs water, swells up, and 

 bursts the perithecium (Fig. 97, D), whereupon its own 

 membrane dehisces, and the ascospores are set free. 

 They at once reproduce the ordinary mycelium of the 

 Fungus. In the case of the Hop, the germinating 

 ascospores infect the young shoot as it first springs up 

 from the soil. In most of the allies of Sphcerotheca each 

 perithecium contains several asci (the product of a 

 single ascogonium), instead of one only. 



Our plant has another means of reproduction, by 

 conidia (see Fig. 97, E). These are produced on vertical 

 hyphse (called the conidiophores) which produce the 

 conidia at the free end. A whole chain of conidia is 

 formed in basipetal order, the oldest thus being at the 

 top. They are detached and scattered by the wind, 

 germinating immediately if they reach a suitable host. 

 They produce mycelium like that from which they sprang, 

 and constitute a ready means of propagation during the 

 summer, while, as we have seen, the ascus -fruits are 

 specially adapted for the winter rest. In some allies 

 of Sphcerotheca, as in the Fungus of the Vine disease, 

 only the conidial fructification is known. 



