On the Nature and Causes of the Decay in Potatoes. 537 
finally, a black or brownish-black colour. When the potato has 
become much diseased, fungi are observed to grow on the diseased 
parts, and they send their mycelium, or spawn, through the cellular 
tissue, and emit abundance of sporules, or germs, which after- 
wards produce other fungi. I wish you distinctly to understand 
that these fungi do not appear at first, and when they do appear 
it is onty on the surface, or on cavities leading to it. 
The appearance to the eye and to the senses you know, unfor- 
tunately, too well to require description : the brightness of the 
surface becomes destroyed, and the part discoloured, sometimes of a 
reddish colour, brown, or black. The disease makes its way from 
the surface to the centre of the tuber, running often into the 
malignant gangrenous form, when the entire structure disappears, 
and the whole becomes a putrid slimy mass of a disgusting odour. 
Now, what are the chemical changes which ensue in the po- 
tato? In the first stages it is difficult to follow them, except by 
an ultimate analysis. In the ultimate analysis of a healthy and 
diseased part of the same potato, the disease being in the first 
stage before the formation of fungi, the only material difference is 
in the diminution of nitrogen. 
The albumen and the gluten must, therefore, have been decom- 
posed. This is found, in after stages of the disease, by the dis- 
tinct smell of ammonia, which is always a product of the decay or 
putrefaction of such nitrogenous matters. At first the starch 
remains unaffected, but as the disease progresses, it also yields to 
the destructive action, and is changed into a soluble gummy mass. 
The small amount of sugar was, doubtless, the first to disappear. 
In one word, the chemical changes are those which invariably 
ensue when a vegetable body passes into a state of putrescence. 
This being a general statement of the condition of the diseased 
tuber, the important questions now arise — What is the disease ? 
and what is its cause ? 
Probably no subject ever enlisted in its consideration such a 
host of inquirers, many of them being men of talent or good prac- 
tical observers, and accustomed to methodical habits of inquiry ; 
others, well-meaning writers who served to complicate the subject 
by speculations thrown out at hazard from evidence contradictory 
and insufficient. Still, notwithstanding all these efforts of mind to 
grasp a hidden evil, the very simplicity of its nature becomes mani- 
fest from the circumstance that only two theories have been pro- 
pounded. One of these theories avers that the source of the evil lies 
in a fungus. The sporules, or seeds of this fungus, have been carried 
by the wind over all the world ; they have entered the plants by the 
breathing pores of the leaves, first causing the stem to wither and 
decay, and so vitiating the juices in the under-ground stem, that 
the tubers receive this unhealthy growth, and become unfit for 
