230 



PRACTICAL BOTANY 



found upon the willow, oak, some of the smartweeds, and 

 upon many other plants. The powdery mycelium lives upon 

 the surfaces of the leaves. Haustoria, by means of which 

 nutrient material is extracted from the host, are sent into the 

 leaf from the superficial hyphse. The fungus is therefore a 

 superficial parasite. 



At times upright hyphse form transverse walls, cutting 

 from their tips rows of small cells, the conidia. The powdery 



appearance of the mildews" 

 is due largely to the pres- 

 ence of large numbers of 

 these conidia. The co- 

 nidia, if favorably placed, 

 are the means of produc- 

 ing new growths of the 

 mildew. Another complex 

 method of reproduction 

 results in forming asco- 

 spores. Two superficial 

 hyphge unite their tips, 

 and fusion of the nuclei 

 of these tip cells takes 

 place. Then there grows, 

 as a result of this fusion, 

 a .relatively large, heavy- 

 walled body, the asco- 

 carp, so called because it is the hard-walled body which con- 

 tains the asci and ascospores. Within the developing asco- 

 carp, division of the tissue finally results in forming several 

 asci, in each of which there are four to eight ascospores 

 (Fig. 186). In late summer the ascocarps may, without mag- 

 nification, be seen as small black bodies upon the surface of 

 lilac leaves. From the walls of the ascocarp peculiar arms 

 extend, and in the lilac mildew and some other kinds these 

 have strikingly branched tips, which sometimes serve as one 

 means of distinguishing the species. 



FIG. 186. The spore-sac case of lilac 

 mildew (Microsphcera alni) 



The central, heavy -walled body (ascocarp) 

 contains the sacs (asci) in which spores are 

 formed. Upon the wall of the ascocarp are 

 stalks, sometimes called arms, which have 

 peculiar branches at their tips. About 60 

 times natural size 



