460 Scientific Proceedings, Royal Dublin Societij. 



almost universal production of " secondary " conidia by the zoospores situated 

 near the edge of the cover glass was observed. 'J'he slide had become con- 

 taminated with bacteria. These new conidia were borne on the ends of long 

 spirally twisted germ tubes. They contained all the protoplasm, even of the 

 branches, where such were present. In form they exactly resembled those 

 produced by conidia, being asymmetrical, and provided wilh prominent papillae 

 and yellow oil-drops, but they were much smaller, although having a con- 

 siderably greater volume than the original zoospore. In a companion pre- 

 paration, which was not contaminated, the zoospores did not produce 

 " secondary " conidia. 



It is evident that the faculty by which germinating zoospores may prolong 

 their existence under unfavourable conditions by developing directly into 

 conidia must be of significance in the life-history of the fungus. The general 

 and rapid production of zoospores by conidia at relatively low temperatures 

 and in subdued light is evidently an adaptation to facilitate leaf infection 

 during the heavy night dews of late summer and autumn. It is probable 

 that under these conditions failure to bring about infection in the night or 

 morning hours results in the death of the spores, at least in dry weather. In 

 the case of conidia which fall to the ground, even though they function as 

 sporangia, it is clear, in the light of our present knowledge, that the resulting 

 zoospores are not nearly so delicate as to lose their vitality (unless they succeed 

 in infecting a potato tuber) a short time after the first shower of rain which 

 brings about their liberation. The conidia produced by zoospores have pro- 

 bably a measure of the same power of resistance as is possessed by other 

 secondary conidia, so that instead of a single body capable of living for days 

 or weeks and then infecting the plant under favourable conditions, the 

 eventual sources of infection are multiplied manifold. The extent to which 

 this multiplication can go was not determined, but " secondary " couidia, the 

 product of zoospores, were seen which were empty and open at the apex as 

 though they, in turn, had agam produced swarm spores. 



Relation of results obtained in microscopical freparations to soil conditions. — 

 The germination of secondary conidia was observed twenty-four days after 

 the original conidia were placed in water, and the germ tubes of the former 

 were still living after thirty-four days. Potatoes were infected many times 

 with conidia which had been fifteen to sixteen days in water (during which 

 time they had germinated, and m some cases produced further conidia), the 

 entry of the germ tubes into thin slices of tubers being observed under the 

 microscope. The hyphae grew between the cells, and fine convoluted 

 branches were sent into the cells and between the starch grains. "Where 

 secondary conidia were not formed, the germ tubes were found to be an 



