152 FUNGOUS DISEASES OF PLANTS 



wall, the remains of the antheridium, the fusion nuclei, and 

 the coenocentrum. The wall of the mature oospore is brown in 

 color and sculptured in a characteristic manner. The oospores 

 are 35-40/x in diameter. It is believed that nuclear division 

 proceeds, during the maturity of the oospores, until about thirty- 

 two nuclei are present, and it has been suggested that each of 

 these divides twice preceding germination, which again takes 

 place by the formation of zoospores. A period of rest is invari- 

 ably required between maturity and germination. 



XI. CYSTOPUS: OTHER SPECIES 



Other species of Cystopus which are very generally distributed, 

 occurring on common hosts of the garden and field, are 



Cystopus Tragopogonis Pers., found on salsify (Tragopogon por- 

 rifolius) and various other Compositse ; 



Cystopus convolvulacearum Otth., on species of the morning 

 glory and sweet potato family, Convolvulaceae ; 



Cystopus Bliti (Biv.) Lev., on several species of pigweed, Ama- 

 rantacea?. The oospores of this species are often very abundant 

 in late autumn ; the stems and flower spikes in which they occur 

 are deformed and usually purplish. 



XII. DOWNY MILDEW OF THE GRAPE 

 Plasmopara Viticola (B. & C.) Berl. & De Toni 



Cornu, M. Etudes sur les Pdronosporees. [Observations sur le Phylloxera 



et sur les parasitaires de la vigne.] 1: 101-184. 1881 ; 2: 1-91. 1882. 

 Farlow, W. G. On the American Grape-Vine Mildew. Built, of the Bussey 



Institution (1876): 415-425. ph. 2-3. 

 Report on Experiments made in 1888 in the Treatment of the Downy Mildew 



and Black Rot of the Grape Vine. Bot. Div., U. S. Dept. Agl. Built. 10 : 



1-61. 1889. 

 Scribner, F. L. The Fungous Diseases of the Grape Vine: I. The Downy 



Mildew. Bot. Div., U. S. Dept. Agl. Built. 2 : 7-18. pis. 7, 2, 4 (in part). 



1886. 

 Viala, P. Mildiou. Les maladies de la vigne (Chap. 2): 57-155. pis. 2-3. 



Jigs. 20-46. 1893. Montpellier et Paris. 



Occurrence. The downy mildew of the grape is one of the 

 most important disease-producing organisms among the Pero- 

 nosporaceae. The fungus seems to be of American origin, and 



