104 



OOMYCETES. 



Family 3. Mycosiphonales. 



The mycelium is bladder-like or branched. Zoospores. Sexual 

 reproduction by oospores, which are produced in oogonia. The 

 latter are fertilised, in some forms, by the antheridium. 



Order 1. Ancylistaceae. The entire bladder-like mycelium is used for the 

 construction of zoosporangia, oogonia, or antheridia. Lagenedium is parasitic 

 on Spirogyra, etc. 



Order 2. Peronosporaceae. Almost entirely parasites. The 

 unicellular, often very long and abundantly branched mycelium 

 lives in the intercellular spaces of living plants, especially in 

 the green portions, and these are more or less destroyed and de- 

 formed in consequence. Special small branches (suction-organs, 



FIG. 88. Polyphagits cuglcna:. A -with smooth, S with thorny oosporcs ; m and /the two 

 conjugating cells. 



" Jiaustoria ") are pushed into the cells in order to abstract 

 nourishment from them. Both oospores and conidia germinate 

 either immediately, or they develope into sporangia with swarm- 

 spores, having always two cilia. Only one oospore is formed in 

 each oogonium ; its contents (Fig. 89) divide into a centrally 

 placed egg-cell and the " periplasm " surrounding it; this is of a 

 paler colour and on the maturity of the oospore forms its thick, 

 brown, external covering. 



The Potato-fungus (PhytopJtthora infestans) is of great interest. 

 Its thallus winters in the Potato-tuber; other organs for pass- 

 ing the winter, such as oospores, are not known. AVhen the tuber 

 germinates, the Fungus-hyphre penetrate the young shoot and 

 keep pace with the aerial growth and development of the plant. 

 The conidiophores emerge through the stomata, especially on the 

 under side of the leaves ; they branch like a tree (Fig. 90), and 



