Murphy — Bionomics of the Conidia of Phytophthora infestans. 463 



At a temperature of 22"-23° C. the limit of development of the conidia 

 is approached. The germination is comparatively poor and germ tubes are 

 the general rule. While abortive sporangia are frequent, the production of 

 perfect zoospores is rare. Exceptionally, zoospores may largely take the place 

 of germ tubes. The reason for this is obscure, but is believed to lie in the 

 conidia themselves. 



Secondary conidia and old but still viable conidia from air resemble each 

 other in possessing a more prominent papilla than usual, and a large yellowish 

 oil-drop. These appear to be signs of their having undergone some maturation 

 process which increases their resistance. Both as a rule produce germ tubes, 

 generally more than one, which may spring from any part of the wall. 



The germination of conidia in water is delayed by lack of oxygen, and 

 may subsequently be brought about by the timely supplying of fresh water. 

 Suspension of germination has been effected by means of bacteria, protozoa) 

 Penicillium spores, thick seeding of conidia, and by ringing cover glasses with 

 paraffin ; and a good measure of germination has thereafter been effected by 

 adding fresh water. 



Secondary conidia, so called, are constantly produced at a certain degree 

 of oxygen concentration which just permits germination, or when the oxygen 

 supply is cut off after germination has taken place. In microscopical prepara- 

 tions they are confined to a zone near the edge of the cover glass, within 

 which germ tubes may live and grow without having the power of forming 

 new conidia. Ungerminated conidia from air are rapidly killed under the 

 latter conditions. 



Secondary conidia appear to be conidia adapted to an aquatic existence, 

 similar bodies being produced by hyphae which fructify in water. They are 

 more resistant than primary conidia, and this resistance is increased by the 

 faculty they possess of successive germination followed by fresh conidium 

 formation, which may be repeated four times. They may also produce 

 zoospores under favourable conditions. 



Zoospore germ tubes may also develop at once into " secondary " conidia 

 when the oxygen supply is reduced after germination has taken place. 

 Zoospore germ tubes may live for at least seven days in water, and their life 

 is further lengthened when new conidia result. The secondary bodies so 

 formed may apparently again produce zoospores. 



The limit of existence of fresh ungerminated conidia in competition with 

 other organisms in water is about five to seven days, and the period may be 

 much shorter. After a germ tube is produced the resistance of the fungus is 

 increased. Potato tubers have frequently been infected with such conidia 

 fifteen to sixteen days after being placed in water. The germination of 



