242 Researches into the Nature of the Potato-Fungus. 
Fig. 4. — Phytoplithora infestans. 
I 
a b • c d 
a. Conidium, in water, with the protoplasm divided to form the zoospores. 6. The zoospores 
escaping from the apex of the conidium. c. The zoospores in motion, d. The zoospores whidi 
have ceased to move, and are beginning to germinate. Magn. 390 diam. 
move freely in the water, like so many swarmspores (^zoospores). 
After a short time these come to rest, and, on a suitable medium, 
each develops a new mycelium. From the facts just mentioned, 
these conidia may be called zoospore-forming cells, or zoo- 
sporangia, a name which is employed^for similar organs observed 
in other water fungi and algze. The zoosporangia of all these 
plants, when disturbed or prevented by any cause from forming 
zoospores, grow at once into a new plant (thallus or mycelium) ; 
and this also often happens in Peronospora and Phytophthora. 
In most species of Peronospora (those called acroblastce, and 
pleuroblastcB) this phenomenon is the rule without exception. 
Zoospores are never formed, but each conidium grows into a 
single mycelium tube. A small number of species (Peronosporce 
plasmatoparce) are intermediate between the two just mentioned, 
but they do not require to be here described. 
The sexual organs and oospores of all the species of Cgstopus 
and Peronospora have essentially the same structure, the charac- 
teristic differences which they possess being of specific, not of 
generic value. The oogonium is a globular cell, completely 
filled with protoplasm, generally the end of a mycelium branch, 
rarely in the middle. Contemporaneously with the oogonium, 
or only a little later, the antheridium belonging to it is developed 
in the end of another mycelium branch, which has already at- 
tached itself firmly to the young oogonium. The antheridium 
is a much smaller cell than the oogonium, and is mostl}' oval, 
or club-shaped. Both organs grow together, closely united, until 
they reach their full size, and then follows the fertilisation of the 
oogonium and the formation of the oospore. In the oogonium a 
thick globular mass of protoplasm, particularly fatty, separates 
itself from the rest. It occupies the central and larger part of 
the cavity ; the periphery is filled with the less compact mass of 
protoplasm. Alter the separation of the protoplasm, the anthe- 
ridium sends out, from the surface applied to the oogonium, a 
