As a further proof that the bitter principle of the hop is distasteful to the aphis, it is noticeable that when the fly first arrives it always attacks the topmost shoots of the bine where the leaves have not developed, and where the active principle is likely to be weakest.  The same position is selected by the aphis of the rose, the bean, and every plant or tree subject to aphis attack—­it is the undeveloped and therefore unprotected part which is chosen.

It is remarkable that when a destructive blight is proceeding—­generally in a wet and cold time—­and a sudden change occurs to really hot dry weather, the hop plant often recovers its tone automatically, shakes off the disease, and the blight dies away, a fact which strengthens the assumption that in normal weather the plant can protect itself.  Again, the blight is always most persistent under the shade of trees or tall hedges, or where the bine is over luxuriant, when owing to the exclusion of light and air the plant is unable to elaborate its natural safeguard.

Fertilizers not well balanced as to their constituents, and containing an excess of nitrogen, act as stimulants without supplying the minerals necessary for perfect health.  The effect is the same as that produced in man by an excess of alcohol and a deficiency of nourishing food, the health of the subject suffers in both cases, leaving a predisposition to disease.

Reasoning by analogy, these causes affecting the success or failure of plants give us the clue to the remedies for bacterial disease in man.  Disease is the consequence and penalty of life under unnatural or unfavourable conditions, which should first receive attention and improvement.  When in spite of improved conditions disease persists, specifics must be sought.  The conditions which produce disease in the vegetable world are fought by the active principle of each plant, and inasmuch as the germ diseases of man are probably, though distantly, related to those which affect vegetable life, the specific protections of plants should be exploited for the treatment of human complaints.  This, of course, has for long been a practice, but possibly more success might be achieved by careful research to identify each distinct bacterial disease in man with its co-related distinct disease in plants, so as to utilize as a remedy for the former the natural protection which the latter indicates.

Our artificially evolved domesticated plants are more subject to disease than their wild prototypes, because they are not natural survivals of the fittest.  They are survivals only by virtue of the art of man, inducing special properties pleasing to man’s senses, and therefore profitable for sale; but in the development of some such special excellence, ability to elaborate protective defence is generally neglected, and the special excellence produced may possibly be antagonistic to the really sound constitution of the plant.  It is thus that cultivated plants are more in need of watchful care and attention than their wild relations, and that, in the development of quality, a sacrifice of quantity may be involved.