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XXXVI. 



ON PURE CULTURES OF PHYTOPHTHORA INFEST AN S 

 DE BAEY, AND THE DEVELOPMENT OF OOSPORES. 



By GEORGE H. PETHYBRIDGE, Ph. D., B.Sc, 



Economic Botanist to the Department of Agriculture and Technical 

 Instruction for Ireland ; 



AND 



PAUL A. MURPHY, A.E.O.Sc.I. 

 Plates XLV, XLVI. 



[Read Febrtjauy 25. Published March 26, 1913.] 

 I. Introductory. 



In endeavouring to ascertain the life-history of any parasitic organism, two 

 ways of approaching the subject present themselves. 



The first, and perhaps the one hitherto most frequently employed, is to 

 obtain the details from the parasite as it grows upon its host. The second is 

 to grow it in pure culture as a saprophyte on a suitable artificial medium, if 

 such can be found, and study its development under such conditions. It is, 

 of course, both theoretically possible, and it has been found to be the case in 

 some instances, that under artificial conditions the parasite may not develop all 

 the stages in its life-history ; but it is also equally possible that under these 

 very conditions the organism may show stages in its cycle of development 

 which are not produced during its career as a parasite. Hence the advantage 

 of approaching the problem by both of the available avenues. 



In the case of Phytoplithora infestans, the parasitic fungus which causes 

 the potato blight, the application of the first method of study has failed so 

 far to reveal with certainty any stage in which sexual organs are produced, 

 although many of its allies amongst the Peronosporaceae have been shown to 

 possess such organs. It is true that certain observers have laid claim to the 



