(55) 



zoospore3 which appear identical with those produced by the conidia. The 

 office of the oospore appears to be to pass the winter. The family consists 

 of two genera, Peronospora and Cj/stujms. The species of tlie former have 

 the appearance of moulds, producing conidia singly or in clusters at the 

 tips of the fertile threads, while those of the latter occur as white pustules 

 on leaves, bearing the conidia in moniliform strings, the fertile hyphae or 

 threads having no prominence. Generally the mycelium of both has curious 

 processes, termed hmipforia, penetrating the cells of the supporting plant, 

 as shown in Plate II, Fig 7. The threads themselves, when first emitted 

 from the spore as well as w'hen buried and ramified in the tissues, often pen- 

 etrate the cell through and through, being found at considerable distances 

 from the diseased-looking spots upon which the fruiting threads appear. 



Over forty species of Peronospora have been described, of which only 

 six are known to exist in the United States. Many others probably await 

 the researches of botanists. Of the six, three, by far the most common 

 ones, are in the present described collection. 



Feronospora infestiuis, Mont., (^Plate II, Fig. 8.) Very common on 

 potatoes and tomatoes. It has also been found on Bittersweet (Solanum 

 dulcamara), and even upon a not closely related plant, Anthoccris viscosa{l'i) 

 belonging to the Scrophulariacesc. This is by far the most famous, or in- 

 famous, of the species of this genus, causing the well known and often 

 dreaded potato rot. Its history has been often toldll^J), but a new chapter 

 was added last year by Worthington Gr. Smith (14), of England. Previously 

 the oospores had not been found as such, and so much search had been 

 made for them that it was quite generally believed that they must exist 

 upon some other supporting plant. Clover, among others, was suspected. 

 The finding of them in the tissues of the leaves and stalks of the potato 

 settled the doubt and bids growers beware of leaving the old potato top to 

 breed the pestilence another year. 



P. gavglifonnis. Berk, {Plate II, Fig. 3.) Common on lettuce and 

 some allied compositse {Lactuca altissima and Nahalus alius. Farlow.) 

 Zoospores not observed. Conidia germinating and penetrating young 

 leaves, the tissues usually decaying from above downward until the whole 

 plant is involved and becomes a slushy, putrescent mass. This parasite is 

 especially destructive in forcing-houses. Several hundred dollars worth of 

 lettuce was lost by one propagator here last winter from this fungus (l-'^>). 

 The only cure now known is to keep the atmosphere as dry as practicable 

 and remove very carefully the diseased leaves. 



P. iiticola, B. & C. {Plate II., Figs. 6 & 7.) Common on grape 

 leaves, and becoming very destructive in this vicinity, worse apparently 



12. Farlow, Synopsis of the Peronosporeae of the United States, Bulletin of 

 the Bussey Institution, p. 426. 



13. Smith, Ohio kg. Report. 1872, Essays, etc., p. 20. 



14. Farlow, Gardener's Monthly, Nov. 1875, p. 274. Smith, Ohio kg. Report, 

 1862, Essays, etc., p 20. 



15. Smith, Gardener's Chronicle, July 17, 1875, p. 69. Quarterly Journal of 

 Microscopic Science, October, 1875. 



16. Farlow on the American Grape Vine Disease. Bulletin Bussey Institution. 

 p. 415. 



