is a couplet repeated in all the hop districts to-day, but the damage done by the flea is not to be compared to that caused by the next pest, the fly.  The latter is one of the numerous species of aphis which begins its attack in the winged state, and after producing wingless green lice in abundance—­which further increase by the process known as “gemmation”—­reappears with wings in the final generation of the lice, and hibernates in readiness for its visitation in the spring next year.

So long as the hop plant maintains its health the aphis is comparatively harmless, for the plant is then able to elaborate to the full the bitter principle which is its natural protection.  On a really hot day in July it is sometimes possible to detect the distinctive scent of the hop quite plainly in walking through the plantation, long before any hops appear, and when this is noticeable very little of the aphis blight can be found.  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.

In the case of the hop the most effective remedy is a solution of quassia and soft soap.  The caustic potash in the soap neutralizes the oily integument of the lice and dries them up, but the quassia supplies a bitter principle not unlike that of the hop, though without its grateful aroma, which acts as a protection in the absence of the bitter of the hop itself.  So closely does the hop bitter resemble that of quassia, that in seasons of hop failure it is said to be employed as a substitute in brewing, and at one time its use for that purpose was prohibited by law.