Murphy — -Bionomics of the Conidia of Pht/tophthora infestans. 451 



organic matter, such as jam or dung, and of too strong sunlight. Tube 

 germination followed by the formation of secondary conidia, while depend- 

 ing to some extent on the conidia themselves, was favoured by light, thick 

 sowing, and by any cause which delayed germination. A temperature factor 

 did not seem to be involved. 



Hallier (9) found that zoospores were formed only when food was lacking, 

 while germ tubes resulted in nutrient solutions. Smorawski (23) apparently 

 arrived at the same result. This conclusion Hecke (10) also largely supports ; 

 but in so far as he found no zoospores produced in solutions containing more 

 than 0-5 per cent, of dry matter, while germ tubes occurred not only in such 

 solutions but were sometimes associated with zoospores in very large numbers 

 in distilled water, he concluded that some peculiarity must lie in the conidia 

 themselves. His conclusion was that very young conidia only (" eben erst 

 gebildete Konidien ") can produce zoospores, but that they do so imperfectly 

 (with partial extrusion of the contents) in nutrient solutions. Older conidia 

 never form zoospores. Their germination is poor in distilled water and 

 generally closes with a secondary conidium, while in nutrient solutions they 

 germinate well. 



Brefeld's (8) conclusion with regard to zoospore production in the 

 Peronosporaceae (including P. infesicms) was that thorough wetting favoured 

 this form of germination. 



McAlpine (14) found that the first crop of conidia produced zoospores, 

 while the later crops were variable. A short period of drying prevented 

 zoospore formation. 



Jones, Giddings and Lutman (12) found that potato juice favoured direct 

 germination and that temperature also affected the process. "At 25° C. 

 more than 50 per cent, of the germinations are by tubes, . . . while at 10° 

 to 20° C. direct germination is exceptional." 



According to Melhus (17) the determining factor does not consist of age 

 or any innate peculiarity of the conidia ; nor of light, the concentration of the 

 solution, or the amount of oxygen available. On the other hand, tempera- 

 ture is all-important. At 13° C. the maximum of zoospore formation is 

 found, while at about 23° C. this has practically ceased and germ tubes are 

 the rule. It is only in extremely strong nutrient solutions (20 per cent, 

 dextrose) that zoospore formation is replaced by some direct germination. 



Fresh experiments on the germination of the conidia. — The results of 

 previous work on the conditions regulating the manner of germination being 

 found so conflicting that no conclusion could be drawn, the problem had to 

 be taken up afresh. Fortunately in the course of this investigation it was 



3c2 



