THE ALG^-rUNGI (PHYCOMYCETES) 219 



species groups of oogonia are formed upon a single hypha. 

 Antlieridial branches come in contact with the oogonia (Fig. 

 179). A tube grows from the antheridium and pierces the 

 wall of the oogonium. Sperms from the antheridium escape 

 and fertilize the eggs, thus forming oospores which can repro- 

 duce the plant. In some cases when Saprohfjnia eggs are 

 not fertilized they develop 

 heavy walls, and after- 

 wards germinate as if they 

 had been fertilized. Such 

 development of eggs into 

 oospores without fertiliza- 

 tion is known as partheno- 

 yenetsis. It is a method of 

 reproduction tluit is met 

 A\ith m some other plants 

 and in some animals. 



209. The grape downy 

 mildew {Plasmopara viti- 

 cold). There are numerous 

 so-called downy mildews ; 

 the one here used as a type 

 frequently appears upon 

 the under surface of leaves 

 of the grape. In the Cen- 

 tral States, which region is 

 supposed to be the original 

 home of this parasite, it has 

 been an injurious pest for many years. Its downy white growth 

 upon the surface of the leaves (Fig. 180) is its most con- 

 spicuous distinguishing characteristic, but it also often grows 

 upon green shoots and fruit. When conditions are thoroughly 

 favorable (proper soil, moisture, and temperature) for the 

 growth of grapevines, the parasite when present may do little 

 damage. At other times it may all but destroy the crop and 

 greatly reduce the vigor of the host plant. 



Fig. 179. Sexual reproduction of a water 

 mold 



Some of the hyphs bear oogonia (o) and an- 



theridia (a) , and "within the oogonia are the 



eggs (e). After Klebs 



