136 Scientific Proceedings. Tlojial Duhlin Society^ 



Penethation into Host. 



As our knowledge stands at present we are left to hypotliesis io account 

 for the entrance of the parasite into the potato-plant. The general assump- 

 tion is that the zoospore, whether it comes from the summer or the resting 

 sporangium, on reaching the potato-tuher, bores its way into the young 

 epidermal cells of the tuber at one of the eyes, and there begins its destructive 

 work. 



Massee (7) holds that the parasite is an epidermal one only, and that 

 when tlie resting spores appear deep-seated, it is due to tlie depression of the 

 infected epidermal cells by the division aroused in the surrounding cells. It 

 is, I think, necessary to distinguish between tlie resting eyes of a ripe 

 tuber in the autumn and those of one sprouting in the spring. The 

 autumn eye is small, dormant, and protected; the spring one exposes its 

 delicate tissue to attack when sprouting, and would moi'e easily fall a victim 

 to marauding zoospores. 



In my winter material I found tubers showing quite small, slightly warty 

 eyes. Examination of these showed them to be full of the parasite, not only 

 of its well-developed resting spores, but of the plasmodium also (PI. IX., 

 figs. 5, 6). Some of the leaves in tlie shoot forming the eye are, as the 

 figure shows, riddled by the resting spores, and practically destroyed. Exa- 

 mination of the cortical cells of the tuber in the region of the eye, and 

 beneath the protecting cork layer, shows that these cells are occupied by 

 the parasitic plasmodium, which is, as Schilberszky states, thus subperidermal. 



The condition of affairs is such that one cannot imagine the parasite can 

 have obtained its thorougli hold of the eye by simple penetration of zoospores 

 from without. It is more reasonable to conclude that (as I have already shown 

 in tlie case of the Spongospora scab (8) of the potato) the black scab, or warty 

 disease of the potato, is propagated, not only by the contact of healthy tubers 

 with spores in the soil, but by the internal passage of the plasmodium from 

 diseased tubers into the potato-tubers formed from them as seed, the next 

 year. On the 6th November, 1908, I inoculated tubers, and slices of tubers, 

 with decoctions and macerations of spores. Tlie tubers were kept in darkness 

 in sand at room temperature. The eyes have sprouted, and though the shoots 

 now (11th March, 1909) show discolouration, it is too early to say if infection 

 has taken place. I also placed a number of warty tubers under somewhat similar 

 conditions, exclusive of the sand. In their case the eyes have sprouted a 

 little ; and the new material in them shows an almost continuous "coating " 

 of the resting spores, which could scarcely have arisen except from within. 

 The vascular tissue of the tuber, in connexion with the eye, was slightly 

 yellow, as if discoloured. It is this last-mentioned mode of propagation, i.e. 



