44, when it made its
appearance in Belgium. The spores produced in myriads were brought over
in the wind, and first attacked the potato crops in the Isle of Wight,
and then spread over the south of England. The course of the disease is
clearly traced from the south of England toward the midland counties, and
all over the island, and into Scotland and Ireland. It was a progress
northward.... This plant, the _Peronospora infestans_, will only grow on
the _Solanum tuberosum_, that is, the cultivated potato.... Just as
plants of higher organization choose their soils, some growing in the
water and some on land, so the _Peronospora infestans_ chooses its host
plant; and its soil is this species, the _Solatium tuberosum_. It will
not grow if it falls on the leaves of the oak or the beech, or on grass,
because that is not its soil, so to speak. Now, the process of growth is
simply this: When the conidia fall on the leaf, they remain there
perfectly innocent and harmless unless they get a supply of water to
enable them to germinate.... The disease makes its appearance in the end
of July or the beginning of August, when we have, generally, very hot
weather. The temperature of the atmosphere is very high, and we have
heavy showers of rain."

The warmth and moisture are, in fact, the conditions necessary for the
germination of the conidia. Their contents (zoospores) are liberated, and
quickly grow in the leaf, and soon permeate every tissue of the plant.

It was clearly established before the committee that not all potatoes
were equally liable to the disease. The liability depends upon strength
of constitution. It is well known that potatoes are usually, almost
invariably, propagated by "sets," that is, by planting tubers, or
portions of tubers, and this method of propagation is analogous to the
propagation of other forms of plants by means of "cuttings." When
potatoes are raised from seed, it is found that some of the "seedlings"
present a strength of constitution which enables them to resist the
disease for some years, even though the subsequent propagation of the
seedling is entirely from "sets." The raising of seedling potatoes is a
tedious process, but the patience of the grower is often rewarded by
success, and I may allude to the fact that the so-called "Champion
potato," raised from seed in the first instance by Mr. Nicoll, in
Forfarshire, and since propagated all over the country, has enjoyed,
deservedly as it would appe