450 Scientific Proceedings, Royal Dublin Society. 



and forty-one hours. In the light of later experience, the possibility that 

 this material was left in the loam from the previous experiment must not be 

 overlooked. The most significant point was that in three cases out of four 

 in both experiments the originally contaminated surface-soil was more 

 pathogenic to potatoes, even after a period of three to three and a half days, 

 during which water equivalent to 162 cm. (approximately) of "rain "had 

 passed through it, than the drainage water was at any time. The longest 

 period of pathogenicity of the surface-soil under the conditions of continuous 

 " rain" slightly exceeded four and a half days. 



II. — Germination of Conidia. 



Previous luorh on the germination of the conidia. — It was conceived that 

 the method by which the conidia germinated might have a bearing on the 

 period of vitality, zoospores being probably more delicate than germ tubes. 

 It was found difficult, however, to deduce from previous work which method 

 might be expected from conidia in the soil. 



While Berkeley and Montague (6) saw the early stages of zoospore 

 formation within the sporangium, and de Payen (20) observed the emptying of 

 the latter, the meaning of the process was unknown to them and to the 

 earlier workers, such as Morren in 1845 (according to Hecke), Schacht (22) 

 and Speerschneider (24), all of whom observed direct germination. ^ It was 

 not until 1860 that the production of zoospores was recorded by de Bary (2) 

 for P. iiifcstans. De Bary (3, 4) was not able to find exactly what the 

 conditions determining the two methods of germination were. He was 

 first of the opinion that germ tubes were formed only when the tubers 

 were infected in the ground, but later found that this did not 

 always hold. He regarded zoospore formation as the normal procedure, 

 and suggested that germ tubes were produced only by old conidia 

 of low germinative power. Strong sunlight prevented the formation of 

 zoospores. A third method of germination described by this author (2, 3, 4) 

 for the first time consisted of tube formation followed at once by the produc- 

 tion of a secondary conidium. De Bary regarded this as a specific mode of 

 germination, due to partial immersion in water. The secondary conidium 

 had the power of producing zoospores, or a tertiary conidium might result. 



Marshall Ward (25), to whom we owe the fullest and most accurate 

 description of the germination process, considered that zoospore formation 

 was favoured by the presence of oxygen in the water and by the absence of 



' That is, the production of germ tubes. When zoospores result, the germination is 

 said to be indirect. 



