ON THE PERONOSPOK^. 187 



3. — Phytophthora. Differs from the last in its multiple and 

 successive conidia, which, when shed, leave swellings on the 

 branches. This section includes P. vifestafis, the Potato-Fungus. 



Although Ferojwspora has become associated in most people's 

 minds with the potato disease, it is well to remember that the 

 potato is only affected by one species of Feronospora, and that 

 other species attack nearly all our ordinary vegetables, such as 

 cabbage, cauliflower, spinach, lettuce, turnip, parsnip, pea, tomato, 

 onion, etc., but fortunately with not the same virulence as in the 

 case of the potato. 



The general characteristics common to all three divisions are, 

 that the ripe conidia, when placed in water, produce zoospores ; 

 which penetrate the plants, and ceasing to move, develop threads, 

 or mycelium. By another, a sexual mode of propagation, the 

 oozonia, after being fertiUsed by the antheridia, produce from 

 their protoplasm a thick-walled oospore. Mycelial threads sprout 

 from this latter, and the above process is repeated. A consider- 

 able period of inactivity may, however, precede the germination of 

 the oospore, which in this case hibernates for the winter, whilst its 

 host decays. The conidia propagate and spread the fungus during 

 the summer, but do not live through the winter. 



The Peronospora^ including Phytophthora^ form a mycelium 

 which is neither so thick nor so gelatinous as Cystopus. The 

 mycelium usually penetrates not only the intercellular passages of 

 the plant which nourishes it, but also perforates the cells them- 

 selves, and in some instances produces little protrusions which 

 press against the cell-walls and become fixed, thus resembling in 

 all respects the suckers which are invariably found on the myce- 

 lium of Cystopus. Long, slender branches of the mycelium 

 emerge into the air through the stomata of the host, at the end of 

 which ellipsoidal conidia are produced. In some cases, these 

 conidia fall off, and at once put out a germinating filament {e.g., 

 P. gangliforjiiis, the lettuce Peronospora, and P. parasitica, the 

 cabbage and turnip Peronospora.) In other cases, the protoplasm 

 first escapes out of the conidium and forms a roundish cell, which 

 at once puts forth a germinating filament {e.g.., P. pygnicBa, the 

 Anemone Peronospora). But quite a different process occurs 

 with other conidia, which are generally of a larger size than those 



