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THE RESTING-SPORES OF THE POTATO DISEASE. 

 By W. G. Smith, F.L.S. 



The potate disease in this country is rarely seen before the month 

 of July, but this year I received some infected leaves for examina- 

 tion from the editors of the " Journal of Horticulture " at the 

 beginning of June, and my reply to the correspondent was printed 

 on June 10. The leaves were badly diseased, and I detected the 

 Peronospora in very small quantities here and there, emerging from 

 the breathing pores. This was a week or ten days before Mr. 

 Berkeley brought the matter before the Scientific Committee of the 

 Eoyal Horticultural Society, and when I heard Mr. Berkeley's re- 

 marks about the Protomyces, I immediately accused myself of great 

 carelessness in possibly overlooking it ; but I was equally certain 

 of the presence of the Peronospora in the specimens I examined. 



On receiving authentic specimens of diseased plants from Mr. 

 Barron, of Chiswick, the brown spots on the potato leaves at once 

 reminded me of the figures of some species of Protomyces, and the 

 dimensions agreed tolerably well with some described plants of that 

 genus, but the spots, when seen under a high power, appeared very 

 unlike any fungus, and they were very sparingly mixed with other 

 bodies much smaller in diameter, and with a greater external 

 resemblance to true fungus spores. These latter spore-like bodies 

 were of two sizes — one transparent and of exactly the same size as 

 the cells of the leaf (and therefore very easily overlooked), and the 

 other dark, reticulated, and much smaller. A few mycelial threads 

 might be seen winding amongst the cellular tissue, and these threads 

 led me to the conclusion that the thickened and discoloured spots 

 were caused by the corrosive action of the mycelium, in the same 

 way as Peach, Almond, Walnut, and other leaves are thickened, 

 blistered, and discoloured by the spawn of the Ascomyces, as 

 illustrated at the last meeting of the Society. 



My opinion, therefore, was soon formed that the " new" potato 

 disease (as it has been called) was no other than the old enemy in 

 disguise, or, in other words, that it was the old Peronospora infes- 

 tans in an unusual and excited condition. That climatic conditions 

 had thrown the growth of this fungus forward and out of season 

 was probable ; but the idea that the pest would not at length attack 

 all and every sort of potato was to me most unreasonable, though 

 the more tender sorts might be the first to suffer. 



Suspecting the two-sized small bodies before mentioned to be of 

 the nature of spores, and remembering my experiments during last 

 autumn with ketchup, in which I observed that the spores of the 

 common mushroom might be boiled several times, and for lengthened 

 periods, without their collapsing or bursting, I thought I would try 

 to set free the presumed spores of the potato leaves by macera- 

 ting the foliage, stems, and tubers in cold water. This maceration 



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