58 



PLANT STKUCTOKES 



dew, Microsphcera, grows on lilac leaves, which, nearly 

 always show the whitish covering after maturity (Fig. 41). 

 The branching hyphse show numerous partition walls, and 

 are not coenocytic as in the Phycomycetes. Small disk-like 

 haustoria penetrate into the superficial cells of the host, 

 anchoring the mycelium and absorbing the cell contents. 



Sporophores arise, which form asexual spores in a pe- 

 culiar way. The end of the sporophore rounds off, almost 

 separating itself from the part below, and becomes a spore 

 or spore-like body. Below this another organizes in the 



same way, then another, until 

 a chain of spores is developed, 

 easily broken apart and scat- 

 tered by the wind. Falling 

 upon other suitable leaves, 

 they germinate and form new 

 mycelia, enabling the fungus 

 to spread rapidly. This meth- 

 od of cutting a branch into 

 sections to form spores is 

 called abstriction, and the 

 spores formed in this way 

 are called conidia, or conidi- 

 ospores (Fig. 43, £). 



At certain times the myce- 

 lium develops special branches 

 which develop sex organs, but 

 they are seldom seen and may 

 not always occur. An oogo- 

 nium and an antheridium, of 

 the usual forms, but probably 

 without organizing gametes, 

 come into contact, and as a 

 result an elaborate structure is developed — the ascocarp, 

 sometimes called the "spore fruit." These ascocarps ap- 

 pear on the lilac leaves as minute dark dots, each one being 



Fig. 41. Lilac leaf covered with mil- 

 dew {Microsplut'ra), the shaded re- 

 gions representing the mycelium, 

 and the black dots the ascocarps.— 

 Caldwell. 



