Murphy — Bionomics of the Conidia of Phytophthora infestans.' 455 



to aeration, presence of other organisms, temperature or nutrient solutions. 

 T'rom a consideration of the frequently different behaviour of two conidia 

 lying close together under apparently similar conditions, one is forced, along 

 with most previous workers, to conclude that different potentialities must 

 reside in the bodies themselves. This was one of de Bary's views, and his 

 suggestion that germ tubes are the result of age and lack of vigour has 

 been endorsed by many of his successors without any convincing proof being 

 adduced to support it. 



Evidence in support of this view can, however, now be giveti. Conidia 

 were tested from an eight-months'-old pure culture on oat agar. The aerial 

 portion of the mycelium at least appeared dead, and it must have been a 

 considerable time since any conidia had been produced. There were, how- 

 ever, some perfect conidia present among a multitude of dead ones. The 

 former germinated in sterile water at 10°-15° C, somewhat slowly but 

 strongly, and entirely by means of germ tubes. A peculiar feature of their 

 germination was that the tube or tubes (often up to five) occasionally sprang 

 from all parts of the wall, although generally they came from the normal 

 place beside the papilla. These conidia were also peculiar in possessing a 

 yellowish oil drop and an unusually prominent papilla, just like secondary 

 conidia. It is conceivable that this is the outward sign of their having gone 

 through some maturation process. 



Similarly with conidia placed in water, whether primary or secondary, if 

 they do not germinate within about twenty -four hours they seem practically 

 incapable of producing zoospores. Here again in most cases (always, indeed, 

 with secondary conidia) we get the same signs of an inward change. Further- 

 more, when such conidia lie long in water (three weeks), the germ tube may 

 arise from any part of the wall. 



An attempt, however, to distinguish between conidia from a comparatively 

 fresh growth which germinate by means of tubes soon after being placed in 

 water at 10°-15'='C., and those with a similar origin which produce zoospores, 

 resulted in failure. Many conidia apparently similar in all respects were 

 marked, some of which afterwards germinated in one way and some in the 

 other. It was observed that only the hyaline conidia germinated at all. 

 Those which were at all dark-coloured or slightly plasmolysed failed to 

 grow. 



III. — Effect of Lack of Oxygen on Development of Conidia. 



Germination delayed or preve)ited hy lack of oxygen. — During the course of 

 these tests it was noted several times that the presence of bacteria or yeasts ' 



