There is however nearly always a small
sprinkling lying in wait, and a few days of unsuitable weather will
reduce the vitality of the plant so that the blight immediately begins
to increase.
There is little doubt that all the distinctive principles of plants or
trees have been evolved, and are in perfect health elaborated, as a
protection from their most destructive insect or fungoid enemies; just
as physical protective equipment, such as thorns, prickles, and
stinging apparatus, is produced by other plants or trees as safeguards
against more powerful foes. If it were not so, plants that are even
now seriously damaged and kept in check by such pests would long ago
have become extinct.
Pursuing this theory it seems likely that the solanin of the potato is
its natural protection against the disease caused by the fungus
_Phytophthora infestans_. The idea is suggested by the invariably
increasing liability to the potato disease experienced as new sorts
become old. The new kinds of potatoes are produced from the seed--not
the tubers--of the old varieties, and the seed, when fully vitalized
and capable of germination, may be assumed to contain the maximum
potentiality for transmission of the active principle to the tubers
immediately descended from it. During the early years of their
existence these revitalized tubers contain so much solanin that they
are not only injurious, but more or less poisonous, to man, and it is
only after they have been cultivated, and have produced further
generations of tubers _from_ tubers, that they become eatable, showing
that in the tuber condition the plant gradually loses its efficient
protection.
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