430 BOTANY OF THE LIVING PLANT 



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 carpogonium. In most of these 

 Fungi there is also propagation by means of minute conidia, which 

 are not sexually produced. They are borne on conidiophores, which 

 are various and characteristic in form. 



The young unicellular ascus has originally two nuclei, which are held to 

 be derived from the paired but not yet fused nuclei of a preceding ferti- 

 lisation. These nuclei fuse as the development proceeds, and the resulting 

 nucleus then divides into two, four, and usually eight (Fig. 362). These 

 divisions are associated with a process of reduction. Each nucleus then 

 becomes a centre of free-cell-formation : an area of cytoplasm round each is 

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

 the spores till ripeness. The spores are liberated either by forcible ejection 

 from the ascus, or frequently by drying up, and rupture of its wall. 



THE MILDEWS (Erysipheae). 



The Mildews are Ascomycetous Fungi, parasitic on the leaves of 

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



FIG. 363. 



c=a germinating conidium of an Erysiphe, showing how the young germ-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 



