66 
MORPHOLOGY 
upon germination produce numerous laterally biciliate zoospores. The 
zoospores germinate promptly, each one sending out a tube that 
penetrates the seedling 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 differentiated 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. 1 60), 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 (.4. Bliti and A. Portulacae), 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 produce laterally biciliate zoospores, 
159 160 
FIGS. 159, 160. Albugo: 159, oogonium showing differentia- 
tion of protoplast into periplasm (containing numerous nuclei) 
and ooplasm (containing one nucleus); 160, oogonium showing 
no such differentiation of protoplast, resulting in multinucleate 
fusion during fertilization; also the multinucleate antheridium in 
contact. 
