Puthybridgk & Laffektv — Disease of Tomato and other Plants. 493 



The absence of antheridia might perhaps be explained by their becoming 

 torn away during the process of teasing out the spores from the diseased 

 tissues, but it seems scarcely probable that all signs of them would thus be 

 obliterated. On the other hand, there is the possibility that the fungus may 

 produce oospores in the tissues of the host parthenogenetically. Phytophthora 

 infestans develops parthenogenetic oospores in pure cultures (8), and at least 

 one other species described as a Phytophthora (P. TJieobromae) produces 

 oospore-like bodies or chlamydospores (?) in the tissues of the host it attacks. 



The disease (as will be seen in a subsequent section of this paper) is 

 contracted from the soil, and it is reasonable to suppose that oospores, or 

 resting spores of some kind, are developed in the dead tissues of the host- 

 We have, however, up to the present failed to obtain convincing evidence that 

 this is the case. That true oospores are produced in pure culture will be 

 evident from what follows in a later section (p. 496). 



IV.— Proof of Pathogenicity. 



Owing to the comparatively sparing prodaction of sporangia and to failure 

 to get the fungus to grow (except to a limited extent) from what possibly 

 were its oospores, under conditions that could be controlled, it was found 

 impossible to obtain the fungus in pure culture by starting from a single 

 spore. It was, however, eventually isolated in the following way : — 



Portions of diseased tomato stems, about one inch long, were steeped in a 

 weak solution of mercuric chloride (1 : 1000) for one minute. After washing 

 in sterile water and allowing to become air-dry, small portions of the inner 

 affected tissues were removed with sterile instruments and under as aseptic 

 conditions as possible to set plates of sterile media such as wort-gelatine, 

 Quaker Oat-agar, and beef extract-agar. 



After a few days, fungus mycelium had grown out from the portions of 

 tissue to the surrounding medium, and fragments of the freshly grown 

 peripheral mycelium were transferred with aseptic precautions to slants of 

 Quaker Oat-agar in test tubes. Some of these transfers, after further growth 

 had occurred, were found to be impure, being usually contaminated with 

 bacteria. In a few cases, however, careful microscopical examination and 

 the subsequent behaviour of the cultures clearly showed that a single 

 organism was present ; and parallel isolations showed that this organism 

 was the same in all cases. 



The organism thus isolated developed sporangia in culture similar to those 

 found on the host plant. Its pathogenicity was proved in the following- 

 way : — Into the stems of each of two healthy tomato plants, grown in pots, 



