66 



MORPHOLOGY 



upon germination produce numerous laterally biciliate zoospores. The 

 zoospores germinate promptly, each one sending out a tube that 

 penetrates the seedhng host and starts a new internal mycelium. 



The sex organs of this genus are formed on the deep mycelium, the 

 oogonia and antheridia appearing on separate hyphae. The oogonium 

 is a globular, multinucleate cell. In the organization of the egg, the 

 protoplast is diflterentiated into a peripheral zone of cytoplasm (peri- 

 plasm), which 

 contains all the 

 nuclei except one, 

 and a central 

 mass of cyto- 

 plasm (ooplasm) 

 containing a soli- 

 tary nucleus for 

 fertilization (fig. 

 159). The anthe- 

 ridium is also a 

 multinucleate cell 

 (fig. 160), which 

 sends out a fer- 

 tilizing tube that 



reaches the egg, and through this the male nuclei are discharged. One 

 male nucleus fuses with the solitary nucleus of the ooplasm, and a 

 heavy-walled oospore is formed. The oospore is liberated by the decay 

 of the surrounding host tissue, and on germination either produces 

 zoospores or develops a mycelium directly. 



In certain other species (A. Bliti and A. Porlulacae), the numerous 

 nuclei of the egg remain distributed throughout its mass (fig. 160), and 

 when the male nuclei are discharged, there is multinucleate fusion, 

 many nuclei pairing and fusing. 



Phytophthora. — P. infestans is the fungus producing potato rot, a disease of 

 great economic importance. The mycelium vegetates in the green parts of the 

 plant, causing wilting and withering of leaves and stem. The sporophores are 

 sent to the surface in immense numbers through stomata, and branch, bearing 

 solitary conidia on the branches. Damp, windy weather is said to spread the 

 disease like wildfire. The mycelium winters in the tubers. 



Plasmopara. — P. viticola is the grape mildew, the groups of branching sporo- 

 phores appearing like downy spots upon the surface of the host (conspicuous on 

 the leaves.) The conidia upon germination develop a mycelium directly, instead 



159 160 



Figs. 159, 160. — Albugo: 159, oogonium showino; differentia- 

 tion of protoplast into periplasm (containing numerous nuclei) 

 and ooplasm (containing one nucleus) ; i6o, oogonium showing 

 no such differentiation of protoplast, resulting in multinucleate 

 fusion during fertilization ; also the multinucleate antheridium in 

 contact. 



