ACCIDENTS, DISEASES, PARASITES. 175- 



coating the inside membrane of the oogonium, contracts 

 after a partition membrane has been formed between the 

 oogonium and the mycelium, and produces a small spherical 

 body inside the oogonium which is termed oosphere. 



Side by side, generally on another branch of mycelium, a 

 small body is formed rather irregular in sh'ape, and filled 

 with granular protoplasm. It is limited by the partition of 

 the tube which bears it; this is the male organ or antheridia* 

 The antheridia gets separated from the mycelium, and, 

 through a special mechanism, which has not been studied in 

 detail for the Plasmopara riticola, the antheridia comes 

 in contact with the oogonium. Its protoplasma passes 

 through the membrane of the oogonium and blends with that 

 of the oosphere without the volume of the latter increasing. 

 After being fertilized in this way the oosphere becomes 

 covered with a cell membrane and develops gradually into an 

 oospore or simply Winter spore, which may grow into a new 

 plant similar to the parent. 



The germination of the spores of mildew is not known, 

 and has not up the present been definitely studied. In the 

 Peronospora family to which the Plasmopara belongs, the 

 spores of these plants germinate like the conidia, and throw 

 out hyphae bearing sporangia or summer spores. 



MOST FAVORABLE CONDITIONS FOB THE DEVELOPMENT 

 OF MILDEW. 



In spring, when the maximum temperature reaches 77 to 

 86 Fahr. and the moisture is sufficient, mildew starts 

 growing. It is more particularly noticeable in places where 

 mildewed leaves have been buried. Although the germina- 

 tion of winter spores has not yet been definitely studied, 

 and consequently the exact conditions required for that 

 germination are not yet known, it is probable that the first 

 germs of the disease conie from them. Millardet* thinks 

 that they attach themselves to young seedlings growing 

 accidentally in vineyards. They would develop on these, and 

 after completing the fructification of conidia, the latter, 

 scattered by the winds, would spread the disease to all the 

 old stumps. 



Although our experiments seem to confirm those upon 

 which Millardet based his hypothesis, the fact does not seem 

 to be constant. We have often found plasmopara on 



* A. Millardet, Nouvelles Recherches sur le Developpement et le Tro.item.ent du Mildioit 

 et de I' Anthracnose Bordeaux, 1887. [Transls.] 



