ASCOMYCKTKS 



419 



after reduction carried out in the nuclear divisions within the ascus. 



In other cases the sexual organs have not been found, and there is 

 reason to believe that normal sexuality has passed into abeyance in 

 many of these parasitic and saprophytic plants, though the asci still 

 remain as morphologically representing the products of the oogonium. 

 In most of these Fungi there is also propagation by means of minute 

 conidia, which are buds not sexually produced. They are borne on 

 conidiophores, which are various and characteristic in form. 



The young unicellular ascus has originally two nuclei, which later fuse. 

 The resulting nucleus then typically undergoes three successive divisions 

 involving a process of reduction. Each of the eight nuclei then becomes 

 a centre of free cell-formation ; an area of cytoplasm around each is 

 delimited by a cell-wall, leaving a residuum of cytoplasm which embeds 

 the spores till ripeness. Ultimately, the spores are liberated, in the 

 majority of cases by forcible ejection, more rarely by disintegration 

 of the ascus-wall. Sometimes the spores are shot out for a distance of 

 a foot. 



The Mildews (Erysiphales). 



The Mildews are Ascomycetous Fungi, parasitic on the leaves of 

 various plants. They take their name from the fact that the patches 



Fig. 317. 



a -a germinating eonidium of an Erysiphe, showing how the young gam-tube 

 at once attaches itself by a haustorium to the epidermis, b -a haustorium in 

 section. (After De Bary, highly magnified.) (From Marshall Ward.) 



of the disease appear white and floury, owing to the formation of 

 their conidia, produced from a cobweb-like mycelium, which grows 

 on the outside of the infected leaf. Its hyphae are attached to the 



