MILDEWS (ERYSIJPHACE^) 



243 



is again relatively common. The mycelium in these parasites 

 develops externally, on the surface of the leaf (Fig. 130, b), the 

 haustoria alone penetrating into the epidermal cells. The 

 mildewed appearance is usually due to the extensive production 

 of chains of gonidia from the ends of unbranched upright hyphae 

 (Fig. 130, c), such chains being very striking in the Mildew 



FIG. 129. a-b, Penicillium. a, small part of mycelium with gonidio- 

 phores ; b, one of the latter enlarged, c-g, Eurotium (Aspergillus). 

 c, very early stage of fruit-formation, showing the coiled hypha (female 

 organ) from which the asci arise ; d, mature fruit ; e, an ascus from 

 the interior of the same ; /, gonidiophore ; g, small part of apex of 

 same, showing the way in which the gonidia are budded off. (a and b 

 after Brefeld ; the remainder after De Bary.) 



commonly found on Forget-me-not leaves (due to a species of 

 Oidium). Later in the season many of these forms develop 



(Sphaerotheca mors-uvcB], the Rose Mildew (5. pannosa), Erysiphe polygoni 

 (on Field Peas and Cucumber), and E. graminis (on Wheat). Many 

 so-called Mildews do not, however, belong to the Ascomycetes, but are 

 Phycomycetes, whose richly branched gonidial-bearing hyphae give a 

 whitish appearance to the leaves ; such are the Cabbage Mildew (Perono- 

 spora parasitica) and Grape Mildew (Plasmopora viticola). 



