Researches into the Nature of the Potato-Fungus. 243 
small beak-shaped projection, which pierces through the walls 
of the oogonium towards the central mass of protoplasm. As 
soon as this projection touches the surface of the globular^mass 
it ceases to grow, and a cellular membrane surrounds the surface 
Fig. 5. — Sexual organs of Peronospora. 
1 
1 and 2. Peronospma AlHruarum, Caspars, on SUUaria media,Srmih. o. Oogonia; n. antheridia 
shortly after fertilisation. A considerable portion of the mycelium iR shown In Fig. 2. 
3. Peronospora grisea, Unger. Oogonium and oospore not quite mature, median (optical) section. 
AU magnified about 390 diam. 
•of the globule. This membrane inci eases in thickness, and 
becomes the inner membrane of the oospore (endosporium) , 
while at the same time there is formed on its outer surface, 
from the peripheral or surrounding protoplasm, the outer mem- 
brane (episporium), which is mostly of a very dull colour, often 
dark. The oospore grows in this way from the globule in the 
oogonium. The walls of the oogonium and the antheridium 
may either assist the mature oospore or wither. 
The mode of germination of the ripe oospore is known in 
Cystopus candidus, and in some species of the PeronosporcB 
pleuroblastcB (P. Valerianellce, P. Alsinearum). In Cystopus the 
oospore produces numerous zoospores; and as regards the forma- 
tion and further development of these all that has been already said 
respecting zoospores holds good. In the species of Peronospora 
just mentioned, each germinating oospore sends out a mycelium 
tube, like that of the conidium belonging to it. It is, therefore, 
quite probable that the species of Peronospora which like 
Tt 2 
