Murphy — Bionomics of the Conidia of Phijtophthora infestans. '159 



a further lease of life, analogous to the survival of mycelium under cir- 

 cumstances which are fatal to fresh conidia. 



Zoospores and the ijrodiwtion of "secondary" conidia. — When conidia 

 germinate by the production of zoospores, the latter, in the presence of 

 sufficient oxygen, settle down after a longer or shorter period of activity, 

 develop a cell wall and immediately put out a germ tube. There is no doubt 

 that oxygen in sufficient quantity favours this whole proceeding. When 

 oxygen is lacking the germination is often abortive, even at favourable 

 temperatures. The zoospores may be more or less differentiated, but none 

 may emerge ; or some or all may escape, but they do not swim far from the 

 sporangium and soon come to rest. Under such conditions it frequently 

 happens that no cell wall is formed, and the zoospore dies and dissolves. 

 That it is lack of oxygen which limits the activity of the spore in these 

 respects is shown by the presence of naked and otherwise abortive zoospores 

 among masses of Phytophthora mycelium. The coming to rest of large 

 numbers of zoospores in a cluster, which is sometimes seen under these 

 conditions, is explainable on similar grounds. The chance coming of one or 

 more into a zone of low oxygen concentration puts an end to their activity, 

 and the accentuation of the dearth of oxygen which their presence causes 

 entraps other zoospores which happen to swim into the locality, and so the 

 process goes on. 



When the zoospores germinate under favourable conditions, the germ 

 tube is of such enormously greater capacity than the original spore, and 

 continues increasing in length (even though not completely filled with 

 protoplasm), or at least remains alive and vigorous-looking so long, that a 

 saprophytic existence is at once suggested (7). However this may be, it is 

 certain that the zoospore germ tubes can live for one week in water, for they 

 have been directly observed to do so for that period. The statement of 

 de Bary (3) that they cannot live longer than twenty-four hours under such 

 conditions is not correct. A single week, however, is not the limit of their 

 existence. They, too, have the faculty of producing " secondary " conidia, 

 an observation which does not seem to have been recorded previously. The 

 conditions are the same as for conidium production by conidia — namely, a 

 reduction in the amount of oxygen. The phenomenon was first observed in 

 a sealed preparation where conidia had discharged zoospores surrounding an 

 air bubble. Six days afterwards there was an attempt at conidium formation 

 by a zoospore germ tube, but it did not come to fruition. Subsequently in a 

 preparation originally set up under sterile conditions and kept for five days 

 at 10°-15^C., in which there had been abundant zoospore formation, the 



