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a thin slice of the potatoes affected with the disease where the decay is 

 confined to minute spots, we see that the still uninjured parts are strikin- 

 gly deficient in starch granules, and that the granules themselves are for 

 the most part smaller and less perfectly formed. The decay com- 

 mences in the walls of the cells which become, more or less brown 

 and moi'e coarsely granulated than in the healthy state of the tissue, and 

 lose their transparency. The process of decay having thus com- 

 menced in the walls of a single cell, is propagated to the walls of the 

 cells contiguous to it, and by this progressive decay a large portion of 

 the tuber is ultimately involved in the disease. I have not been able 

 to detect any fungus or mycelium of fungus in the tubers either of the 

 last or the present year, and I would attribute the decay, not to the 

 development of fungi, but to the same cause that gives rise to the de- 

 cay in apples, and other fruits, the diminution or total loss of the 

 vitality of the cell. That fungi are afterwards developed in the tubers 

 as in any other dead vegetable matters, is indisputable, since they 

 have been seen by such distinguished microscopical observers and 

 mycologists as the Rev. M. J. Berkeley, and Mr. Stevens of Bristol ; 

 but that they are necessary concomitants to, or causes of, the disease, 

 I feel inclined to deny, since in none that 1 have examined were they 

 present, and as I used an achromatic microscope of such power as to 

 make the larger starch granules appear a quarter of an inch in dia- 

 meter, I think that I should have readily detected them, had they been 

 present. It would not however be difficult to mistake the junctions 

 of the hexagonal cells for the mycelia, as they put on somewhat the 

 appearance of tubes; but as these are present in the perfectly healthy 

 as well as in the diseased potato, they cannot be considered as 

 mycelia. 



On the supposition of the presence and direct influence of fungi in 

 the production of decay, it would seem improbable that decay should 

 take place to so great an extent as I have observed, without the full 

 development of the fungus, which was not the case in the specimens 

 I have examined. Fi"om all I have stated above I shall draw the con- 

 clusion that the production of fungi in the different parts of the 

 potato plant, is the concomitant rather than the cause of the disease. 



Let us now endeavour to trace the disease in a more systematic 

 manner, and first of the causes which may bring about the disease. 

 The tracing of chains of causation is at all times difficult, even when 

 we have to deal with the more simple bodies of the inorganic king- 

 dom, or even the materials of which organized beings are formed ; 

 but when a question arises in which the laws of vitality are involved, 

 Vol II. 4 k 



