EXPEEIMENTS WITH KEFERENCE TO THE POTATO DISEASE. 63 



the two previous seasons. The skin of the under-ground 

 parts of the stems, at first only discoloured, had a withered 

 appearance wlien examined in tlie middle of Jiuie ; and about 

 that period some spots began to appear on the foliage. 



It may be remarked that the above indications were similar 

 to those observed in the preceding season ; and it was fully 

 expected that the stems and foliage would rot as soon as wet 

 weather favoured the process ; for such result followed similar 

 symptoms in 1846. In that year, with disease producing visible 

 effects on the plants below ground, and doubtless pervading 

 their whole vegetation, it was not expected that the stems would 

 be able to continue a supply of nourishment to the tubers ; and 

 in the end of June an attempt was made, by additional earthing 

 up, to induce fresh roots above the decaying under-ground part 

 of the stems. A few roots were produced, but insufficient to re- 

 invigorate the bulky, but unsound, tops. From continued de- 

 prival of adequate nourishment, and from inherent local disease, 

 the stems subsequently flagged during a period of heat and 

 drought. Dryness not being conducive to the rotting of vege- 

 tables, the potato stems lingered till they became surcharged 

 with moisture, admitted through diseased tissue. In a few days 

 the whole of the stems and foliage was completely disorganized, 

 and became a mass of putrefliction. 



Judging from a comparison of symptoms and extent of disease 

 at the beginning of July, in tliis and the preceding crop, it cer- 

 tainly was not anticipated that the plants would exhibit any 

 signs of decidedly healthy vegetation during the remaining 

 period of their existence, in the present season. In the course 

 of last July, from prostrated stems, some very fresh healthy 

 foliage was however produced ; healthy tissue was in many 

 instances protruded over the cankered portions of the ujider- 

 ground parts of the stems ; and fresh roots were also then emitted. 

 Nothing like this healthy action was evinced by potato plants 

 in 1845 and 1846, in the Garden of the Society, at a similar 

 period of their groA\'th. 



The foliage, which resulted from an apparently fresh start of 

 growtli in July, was of a lively green, very different from tliat 

 yellowish tint which characterized the young foliage produced at 

 a similar period in the two former seasons. From this fresh 

 foliage the healthy substance must have returned vuiconfaminated 

 along the stems till it reached the previously diseased parts on 

 the under-ground portion of stem. It had then no longer any 

 direct cliannels ; and like the organized matter from the base of 

 a cutting, it was excreted in the form of comparatively naked 

 tissue. Even in this state it seemed not affected by the diseased 

 surface over which it progressed ; and this leads to the conclu- 



