104 
THE POTATO DISEASE. 
they depend partly on local or individual circumstances, which 
may be obviated, and which influence some epidemics more than 
others, and partly to general influences, commonly supposed to 
be atmospheric, but regarding which we actually know nothing 
but their existence. There is a striking general resemblance 
between the rise and progress of epidemics and the appearance, 
non-appearance, and increase of fungi from season to season. 
Coupling this analogy with the opinion generally gaining ground, 
that certain epidemics owe their existence to the growth of fungi, 
or analogous beings in the animals afflicted, Mr. Goodsir con¬ 
ceived that we are bound, in our attempts to explain the nature 
of the potato disease, not to overlook the fungi which exist in the 
diseased tubers. 35 
After some further remarks on the structure of this and other 
fungoid substances, Dr. Maclagan remarked, “ that the question 
as to the nature of the disease in the potato was not settled by 
proving the presence of a fungus in the altered portions. It was 
still a disputable point whether the fungus was antecedent to or 
consequent upon the morbid state of the tubers ; it was yet 
doubtful whether the discrimination of the first advances towards 
the disease fell within the province of the chemist or the botanist. 
He had frequently observed, on making sections of affected po¬ 
tatoes, portions in the interior of the tubers, in which no disco¬ 
loration had commenced, but which were in a soft, pulpy condi¬ 
tion . A portion of this could at once be lifted out on the point 
of the knife, and on being subjected to microscopic examination 
no fungus or brown granular matter could be observed ; but th 
amyliferous cells of the tuber, and those containing starch grains, 
were found in a swollen state, as if they had been filled with fluid 
by endosmose, and the component parts of the cellular tissue had 
thus become so entirely detached from each other, as to have 
assumed a complete round form, instead of their characteristic 
hexagonal shape. It appeared to him to resemble what might 
be expected from the maceration of the textures in water, and it 
was a possible supposition that this might be the first stage of 
the disease, and that the change thus effected in the tuber formed 
a nidus for the development and growth of the fungus already 
existing in the aerial part of the plants. Mr. Milne said nothing 
could be more distinct than the description given of the nature 
