carefully, he will perhaps find that there is scarcely a plant which is not spotted.  If the thunder shower which we have imagined be followed by a long period of drought, the plague may be stayed and the potatoes saved; but if the damp weather continue, the number of spotted leaves among the potatoes increases day by day, until the spotted leaves are the majority; and then the haulm dies, gets slimy, and emits a characteristic odor; and it will be found that the tubers beneath the soil are but half developed, and impregnated with the disease to an extent which destroys their value.

Now, the essential cause of the potato disease is perfectly well understood.  It is parasitical, the parasite being a fungus, the Peronospora infestans, which grows at the expense of the leaves, stems, and tubers of the plant until it destroys their vitality.  If a diseased potato leaf be examined with the naked eye, it will be seen that, on the upper surface, there is an irregular brownish black spot, and if the under surface of the leaf be looked at carefully, the brown spot is also visible, but it will be seen to be covered with a very faint white bloom, due to the growth of the fungus from the microscopic openings or “stomata,” which exist in large numbers on the under surface of most green leaves.  The microscope shows this “bloom” to be due to the protrusion of the fungus in the manner stated, and on the free ends of the minute branches are developed tiny egg shaped vessels, called “conidia,” in which are developed countless “spores,” each one of which is theoretically capable of infecting neighboring plants.

Now, it is right to say that, with respect to the mode of spread of the disease, scientific men are not quite agreed.  All admit that it may be conveyed by contact, that one leaf may infect its neighbors, and that birds, flies, rabbits, and other ground game may carry the disease from one plant to another and from one crop to another.  This is insufficient to account for the sudden onset and the wide extent of potato “epidemics,” which usually attack whole districts at “one fell swoop.”  Some of those best qualified to judge believe that the spores are carried through the air, and I am myself inclined to trust in the opinion expressed by Mr. William Carruthers, F.R.S., before the select committee on the potato crop, in 1880.  Mr. Carruthers’ great scientific attainments, and his position as the head of the botanical department of the British Museum, and as the consulting naturalist of the Royal Agricultural Society, at least demand that his opinion should be received with the greatest respect and consideration.  Mr. Carruthers said (report on the potato crop, presented to the House of Commons, July 9, 1880, question 143 et seq.):  “The disease, I believe, did not exist at all in Europe before 1844....  Many diseases had been observed; many injuries to potatoes had been observed and carefully described before 1844; but this particular disease had not.