228 Scientific Proceedings^ Royal Dublin Society. 



fungus living in such shoots that the infection is derived whicli, under 

 suitable vi^eather conditions, develops into an epidemic. 



During his investigations of the various diseases of the potato, 

 Dr. Pethybridge succeeded in detecting at least one new parasite, and in 

 adding considerably to our knowledge of several others which contribute to 

 the losses sustained by this important crop. The new (and by far the most 

 important) one was named PhytopMhora erythroseptica. Not only does this 

 parasite cause great losses — sometimes greater even than those caused by 

 P. infestans, and hence the importance of its discovery from a practical point 

 of view — but also its discovery and investigation have contributed funda- 

 mentally to our knowledge of P. infestans, and has brought to light a method 

 of development of sexual organs which up to that time was quite unique in 

 the vegetable world. 



De Bary has described the fertilization of certain fungi related to the 

 Phytophthoras. In these the female germ-plasm is contained in a globular 

 capsule, the oogonium. Close to this there arises a club-shaped capsule, the 

 antheridium, containing the male germ-plasm. A beak is formed on the 

 antheridium which applies itself to the oogonium and penetrates it. Through 

 this beak the male germ-plasm passes into the female germ-plasm, and so 

 effects fertilization. This process is known in several parasites described as 

 species of Phytophthora, and no one doubted that the same process would be 

 found to take place in P. infestans if and when the sexual organs of this 

 species were discovered. 



It was, therefore, with no little surprise that Dr. Pethybridge found in 

 his cultures of P. erythroseptica that the developing oogonia grow thruttgh the 

 young antheridia, and that the oospore is formed in the oogonium after its 

 emergence on the far side of the antheridium. Such a penetration of the 

 male organ by the female had till then been quite unknown. 



Having established this peculiar and novel type of development for what 

 we may call Dr. Pethybridge's fungus, he next turned his attention to the 

 origin of the oospore in P. infestans. For it must be stated now that mean- 

 while in the United States of America, Dr. Clinton had succeeded in 

 forcing this parasite to produce sexual organs, not, indeed, in any part of 

 the living potato plant, but as a saprophyte in pure culture in vitro. Dr. 

 Pethybridge, with his assistant, Mr. P. A. Murphy, using a slight modifica- 

 tion of Clinton's medium, succeeded in confirming the fact of oospore 

 formation in P. infestoAis, but was able to show in addition the important 

 fact that the method of development followed was not that which had 

 been expected, but was identical with the one which he had just previously 

 discovered in P. erythroseptica. Since then, in Dr. Pethybridge's laboratory, 



