hen the gardeners and
farmers are all looking anxiously to the progress of their crops, how
often have we heard the congratulatory remark of "How well and strong
those potatoes look!" Such a remark is most common at the end of July or
the beginning of August, when the green part, or haulm, of the plant is
looking its best, and when the rows of potatoes, with their elegant rich
foliage and bunches of blossom, have an appearance which would almost
merit their admission to the flower border. The same evening, it may be,
there comes a prolonged thunder storm, followed by a period of hot,
close, moist, muggy weather. Four-and-twenty hours later, the hapless
gardener notices that certain of his potato plants have dark spots upon
some of their leaves. This, he knows too well, is the "plague spot," and
if he examine his plants 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