134 SYSTEMATIC BOTANY. 



the zoospores. The spermatozoids are produced in small 

 cells which have arisen by simple fissiou of one of the 

 larger cells. 



167. Other representatives of the Oosporese are the uni- 

 cellular Saprolegnia, and allied genera, which grow on dead 

 fishes, cray-fishes, etc., and may be found parasitic on 

 young, living fish in aquaria. They are often extremely 

 abundant, and then cause immense losses in fish breeding. 

 They multiply uon-sexually by the production of ciliated 

 zoospores, which are formed from the protoplasm in the 

 end of a branch. The zoospores swim about for a few 

 minutes, come to rest, their cilia disappear, and after a few 

 hours they germinate by sending out a filament which 

 produces a new plant. The sexual reproduction is by 

 means of oogonia and antheridia. When the two arise 

 on the same plant, fertilization takes place by direct con- 

 tact of the antheridia with the oogonia, and the passage 

 of the contents of the former into the latter through the 

 tubular process of the antheridium. When the plants are 

 dioecious, motile spermatozoids are produced for the pur- 

 pose of fertilization. The oospores have a thick, double 

 integument (exospore and endospore), and germinate by 

 sending out a tube after a period of rest. 



168. The Peronosporeae (division Oosporese) deserve 

 special attention on account of the great damage they do 

 by their parasitism on living plants, and many of these are 

 common in cultivation, as the Potato, Clover, Spinach, 

 Grape- Vine, etc. The branching mycelium is unicellular, 

 and grows into the tissue of the higher plants, sending 

 minute branches, called haiidoria, into the cells themselves 

 for nourishment. They multiply with great rapidity by 

 non-sexual spores, called conidia. In the genus Oystopvs, 

 which may be seen on Shepherd's-Purse, and other Crucir 



