Ch. X] THE BLIGHTS 437 



they are carried, and form zoospores, which move over the 

 wet surface, come to rest, send hyphae through stomata, 

 and thus start the disease anew. The sexual reproduction 

 takes place deep in the tissues, where a sperm nucleus from 

 an antheridium unites with an egg nucleus. There results 

 a resting oospore which is ultimately set free by decay of 

 the tissues, and germinates in the spring to zoospores which 

 infest young growing plants of the suitable kinds. 



Belonging to this group is the Potato Blight or Rot (Phyto- 

 phthora infestans) which destroys potato tops, and hibernates 

 in the tubers. It sends its conidia, on long-stalked branch- 

 ing sporophores, out through the stomata, whence they are 

 disseminated by wind ; and as they germinate to zoospores 

 on other wet leaves the disease spreads rapidly in warm, wet, 

 windy weather. Sexual reproduction has not yet been found 

 in this parasite. The Grape Blight, or "False Mildew" 

 (Plasmopara) , of which the branching sporophores form 

 downy spots on grape leaves, does very great damage in 

 some places unless held in check by Bordeaux mixture, of 

 which the copper salts are effective fungicides. The Damping- 

 off Fungus (Pythium species), which causes rapid wilting of 

 young seedlings in greenhouses when kept too damp and 

 unventilated, is also of this group, as are the Onion Mold 

 and some other plant parasites. 



Order 3. Mucorales (Zygomycetes) : the True Molds or 

 Black Molds. These comprise the familiar black-headed 

 Molds which appear upon damp bread, and also upon pre- 

 serves and various decaying and refuse organic matters, 

 though blue and green Molds belong to other groups. So 

 ubiquitous are they, that one can at any time obtain a 

 laboratory supply from damp bread kept a few days under 

 a shaded bell jar. The plant body is a ccenocytic mycelium, 

 of which the white or glistening hyphae branch so profusely 

 over the substratum as to present often a woolly or cob- 

 webby appearance. They form many small dark-colored 



