THE MILDEWS 



249 



which are formed in chains from the ends of erect hyphse (Fig. 

 219, A) and give the leaves a powdery appearance. But the 

 most important fructifications are the sac fruits (ascocarps), 

 which appear later in the season as black dots on the leaves. 

 They can be most conveniently studied in the lilac mildew. 



The lilac mildew. This type (Microsphcera Alni) forms white 

 blotches on the leaves of the lilac, especially over somewhat 



FIG. 219. Reproductive organs of the mildews 



A, B, the lilac mildew (Microsphssra Alni) : A, a chain of air spores (conidia) 

 formed from the tip of an erect filament; B, a sac fruit (ascocarp) cracked 

 open, with two spore sacs (asci) protruding, one of the appendages shown 

 in detail. C, D (Podosphsera) : C, the sexual organs, a the antheridium, b 

 the female gamete or ascogonium ; D, the development of the cellular envel- 

 ope of the sac fruit around the fertilized female gamete. C, D, after Harper 



shaded portions of the plant. The sac fruits are found in the 

 autumn as black globular bodies made up of filaments so closely 

 united that they form a cellular mass (Fig. 219, B), in the 

 interior of which are developed the spore sacs (asci). The sac 

 fruit of Microsphcera has several radiating appendages with 

 peculiar tips. It is developed as the result of a sexual process 

 involving the fusion of two sexual cells, or gametes (Fig. 219, C), 

 The asci are formed at the ends of hyphse that arise from the 



