roTATo-ruNGUS. 153 



of May, there was nothing rt markable in the weather ; it was, in general, 

 moderately moist. The wet weather, under the influence of which 

 the fields here suffered so much from FhytophtJiora, did not come till 

 much later ; and at the time when my experiment was completed, I 

 did not, in a number of excursions specially made for the purpose, dis- 

 cover any PhytopJdlwra in tlie fields. The garden in which the ex- 

 periment was made was in the town, far from the fields ; it is to be 

 hoped my experiment was not the means of extending the infection to 

 the fields. 



The results I have described having been accurately ascertained, 

 the problem before us is as far as possible solved ; that is to say, I 

 have shown that the oospores are not foufid in this district, and that 

 the perennial mycelium discharges the functiou of hibernation which 

 is proper to the oospores in other species. 



I may in a few words draw attention to this fact, that the 

 generally-known phenomena connected with the occurrence of the 

 Fungus correspond completely with the result at which I have 

 arrived. This may not at first sight appear to be the case, for while 

 the first infection of the plants in the fields takes place, as we see, in 

 spring, the occurrence of PhytopJdhora is seldom plainly visible before 

 July. But then even in large fields there can hardly be more than a 

 few original seats of infection, since comparatively few diseased 

 Potatoes will be planted, while numbers of the diseased tubers actually 

 planted are rendered harmless from the Fungus not forming conidia 

 either in them or in their shoots. There must be a very large quantity 

 of conidia to enable the Fungus to spread over extensive areas. A 

 comparatively small quantity can only be produced at first in the pri- 

 mary seat of infection. The original development of the Fungus, and 

 the production of secondary centres of infection, must therefore pro- 

 ceed slowly and unnoticed. That is to say, the Fungus needs time to 

 acquire the quantity of reproductive conidia necessary to affect large 

 areas. Were it not so, the Potato-plant in damp places or in damp 

 years would be attacked in spring and perish. It may not be super- 

 fluous to say that the case could scarcely be different if the Fungus 

 hibernated by means of oospores which germinate in spring. Of course, 

 if they were of rare occurrence, then the actual state of things at 

 present would be brought about; but if they occurred very frequently, 

 then an immediate and general attack of the disease in the spring 

 would be unavoidable. 



I have only to add to what I have said, that the vegetation of 

 Phytophthora is known to be largely hastened and assisted by damp, and, 

 on the other hand, to be retarded by drought. As a rule, however, 

 the period of its first appearance is followed by the dry season of the 

 summer. The period of its spreading extensively usually coincides 

 with the beginning of the wet autumn, or, as was to be seen here in 

 1875, with wet seasons in the height of summer. In particular 

 localities it may even occur on the plant before the arrival of these 

 seasons, as in the valleys among our mountains, where there are 

 regular heavy dews and comparatively numerous showers. In con- 

 clusion, attention should also be directed to the possibility of there 

 being a connection between the phenomena in question, and the fact 

 that the Potato-plant in its various stages of development supplies a 



