108 rOTATO-FUNGUS. 



Fi;^. 4. — I'lnjtfiphthoia infcstniis. 



a. Conidium, in water, with tho protoplasm divided to form tho zoospon-s 

 />. The zoospores escapinj^ from the apex of tho conidium. c. The zoospores in 

 motion, d. The zoospores -which have ceased to move, and are beginning to 

 germinate. Magn. 390 diam. 



like so many swarmspores (zoospores). After a short time these come 

 to rest, and, on a suitable medium, each developed a new mycelium. 

 From the facts just mentioned, these conidia may be called zoospore- 

 forming cells, or zoosporangia, a name which is employed for similar 

 organs observed in other water Fungi and Algsc. The zoosporangia of 

 all these plants, when disturbed or prevented by any cause from form- 

 ing zoospores, grow at once into a new plant (thallus or mycelium) ; 

 and this also often happens in Peronoapora and Fhytophthora. 



In most species of Pero7iospora (those called acrohlastce and pleuro- 

 llastcB) 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 {Peronospora plasmatoparai) are in- 

 termediate between the two just mentioned, but they do not require 

 to be here described. 



The sexual organs and oospores of all the species of "Cystopus and 

 Peronospora have essentially the same structure, the characteristic 

 differences which they possess being of specific not oi generic value. The 

 oogonium is a globular cell, completely filled with protoplasm, gene- 

 rally the end of a mycelium branch, rarely in the middle. Contem- 

 poraneously 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 attached itself firmly to the young oogonium. The 

 antheridium is a much smaller cell than the oogonium, and is mostly 

 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. After 

 the separation of the protoplasm, the antheridium sends out, from the 

 surface applied to the oogonium, a 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 sur- 

 rounds the surface of the globule. This membrane increases in thick- 

 ness, and becomes the inner membrane of the oospore (endosporium) ; 



