P0TAT0-FTTNQU8. 115 



the tuber, while the Fungus generally keeps to^ the periphery 

 or exterior portion, where it discolours and kills the cells. During 

 the sprouting of the tuber we can see the Fungus shooting from the 

 discoloured portion into the watery and healthy centre, where it grows 

 very luxuriantly. It sends out many branches between the cells of 

 the tuber, and also forces short branches into the interior of the cells ; 

 it is vigorous, filled with colourless protoplasm, and gives an im- 

 pression of most exuberant growth No discoloration of the watery- 

 tissues, it should be said, takes place. These phenomena suggested 

 the possibility that the luxuriant branching of the Fungus in the 

 sprouting Potato was for the purpose of forming the oospores. Oa this 

 hypothesis hinged another, viz., that the supposed formation of 

 oospores must be completed, and the oospores matured, simultaneously 

 with the shrivelling up of the seed-tubers. This would, moreo "er, 

 be about the time of the full development of the Potato-plant, when 

 the Fungus usually appears in large quantities. Now, it is not abso- 

 lutely necessary that oospores should pass a winter before germination ; 

 the germination, as has been stated above in regard to Saprolegniece, 

 may take place speedily after they arrive at maturity ; and in the 

 Peronosporece it may at least be looked upon as a possible phenomenon, 

 depending upon surrounding circumstances. Thus arose the conjec- 

 tures that perhaps the oospores of the Potato-fungus originated from 

 the mycelium growing in the sprouting tuber ; that the oospores ger- 

 minate immediately after they reach maturity, which is con- 

 temporaneous with the fuU'developmentof the foliage ; and that their 

 germs at once attack the foliage. The difficulty in the way of 

 accepting this theory, because the tuber is under while the foliage is 

 above the ground, is easily set aside when we remember that the 

 sprouting tuber is almost always sought after by minute animals — 

 Acarus, Juhis, and Liimhricics — by whose agency the oospores could 

 readily be brought to the surface of the soil. 



I must begin by saying that the researches made for the purpose 

 of testing these theories have also been followed by purely negative 

 results. Still it will not be useless if I describe briefly the various 

 steps of the experiments. 



At the end of February and beginning of March a considerable 

 number of Potatoes, up to then healthy, were artificially inoculated, 

 varieties being selected which were known to have remained a long 

 time watery-turgescent after sprouting. The inoculation took place in 

 this way : fresh conidia, capable of germinating, were placed on the 

 terminal eyes of the tuber ; the infected spot was then covered with 

 a piece of wet blotting-paper, and the tubers were placed in a moist 

 atmosphere (under a glass bell), out of which they were not taken for 

 several days. By this process infection can be obtained with great 

 certainty ; the existence of the Fungus is, after some time, clearly 

 visible externally by the browning of the eyes and the sinking ot the 

 surrounding parts. In continuing the experiment, only those tubers 

 were made use of in which infection had actually taken place — about 

 sixty in number — and the microscopic examination which followed 

 invariably disclosed the presence of the Fungus in their interior. At 

 the end of March and beginning of April they were planted partly in 

 fiower-pots and partly in the open ground. From the eyes which 



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