POTATO-FUNGUS. 1 1 1 



epidermis, which they burst open by the increasing accumulation of 

 conidia. In Peronos'pora and Phytophthora they penetrate the epi- 

 dermis into the air, either singly or in small bunches, mostly through 

 the stomata ; in parts where these are absent they pierce through the 

 cells of the epidermis. The mature conidia fall from their pedicels, 

 and being light bodies are readily carried away by currents of air or 

 by minute animals. As soon as the conidia are mature they are ready to 

 germinate, and do indeed germinate when surrounded with moisture, 

 and form zoospores when they are placed in water. The germinating 

 spores, when they find a suitable host, at once penetrate the epidermis 

 into the interior of the plant, and grow into new mycelium, which soon 

 again pushes into the air and forms more conidia. A short time and 

 a small quantity of water are all that is needed for the germination of 

 these spores and their penetrating the host-plant. A single damp 

 night or a short shower furnish the necessary conditions. After a few 

 days the new mycelium sends out its first conidia, and these continue 

 to be followed by others for weeks. These phenomena explain why 

 it is that the Fungus in question is able to spread itself in summer 

 rapidly over large districts and attack many plants. The conidia are 

 the organs of this propagation. They do not retain their vitality and 

 power of growth for more than a few weeks at most. That they re- 

 main alive through the winter has in no case been observed. 



The oospores of Cystopus and Peronospora are formed in the inte- 

 rior of the tissue of the host, mostly between the cells, rarely in the 

 interior of the cells, as, for instance, in Peronospora Sempervivi, dis- 

 covered by Professor Schenck. They have their origin either in the 

 same part of the host where the conidia occur, or their formation is 

 limited to a particular spot. The species of Perononpora which in- 

 habit different kinds of host-plants are found on all of them, or at least 

 on several of them, with conidia as well as with oospores, e.g., P. AUi- 

 neariim on species of Stellaria and Cerastium, P. Ficarice on several 

 s'peGies\oi Ranunculus ; or theyform conidia on all hosts, but oospores only 

 on certain hosts, e.g., Cystopus cuhicus. "When therefore a species of P^ro- 

 nospora is always found in a phanerogamic plant with conidia only and 

 without oospores, it is fair to conjecture that the Fungus inhabits 

 also another host-plant 'in which the oospores must be souo-ht. 

 This conjecture must be based, not on a single instance, but on extended 

 and long observation, for both the presence and the absence of oospores 

 may be due to other causes, such as climate and season. I find P. 

 Alsinearum in the country here on Stellaria media always with 

 oospores and conidia in spring, while in autumn often with conidia 

 but never with oospores. In Cystopus Candidas, the well-known Rust- 

 fungus of the Cruciferce, the oospores occur very seldom in North Ger- 

 many, so far as my experience goes, but are common in South-west 

 Germany in many species of host-plants. "We have not then abso- 

 lute laws to deal with here, but laws which in special instances admit 

 of exceptions. For example, Cystopus cuhicus occurs very often on 

 species of Tragopogon, Podospermum, and Scor%onera. I have found it 

 almost always with conidia and oospores in Sc. hispanica, but for years 

 as regularly without oospores on all other hosts. In one case, how- 

 ever, I found the oospores on the leaves of Tragopogon porrifolius. 



"When the oospores are mature, the part of the host which contains 



