The proper study of plant pathology presupposes and involves a knowledge of the physiology of plants, of the normal relations of the latter to their environment, and of the biology of those animals and plants (principally insects and fungi) which are parasitic on them. It is of the first importance to understand that a disease is a condition of abnormal physiology, and that the boundary lines between health and ill-health are vague and difficult to define. As with the study of the diseases of man and other animals, so with those of plants, the practice resolves itself into the accurate observation and interpretation of symptoms (_Diagnosis_) on the one hand, and of causes (_Aetiology_) on the other, before any conclusions of value can be drawn as to preventive or remedial measures (_Therapeutics_). In plants, however, symptoms of disease are apt to exhibit themselves in a very general manner, or at any rate it may be that our perceptions of them differentiate symptoms due to very different reactions imperfectly, probably because the organisation of the plant is less specialised than that of animals. The turning yellow and premature falling of leaves, for instance, is a frequent symptom of disease; but it may be due to a long series of different causes of ill-health--_e.g._ drought, too high or too low a temperature, light of insufficient or of excessive intensity, a superfluity of water at the roots, the presence in the tissues of parasitic fungi, or that of worms or insects at the roots or elsewhere, poisonous gases in the air, soil, etc., and so forth. Consequently the science of plant pathology is much concerned with the direct action of external causes, which are probably less obscure than in the case of animals, though by no means always obvious. Such considerations at any rate seem to account for the fact that most authorities on plant pathology base their classification on the causes of disease, there being few noteworthy exceptions. NOTES TO CHAPTER IX. The bibliography here quoted will be found in Berkeley, "Vegetable Pathology," _Gardener's Chronicle_, 1854, p. 4; Plowright, _British Uredineæ and Ustilagineæ_, 1889; Eriksson and Henning, _Die Getreideroste_, Stockholm, 1896; De Bary, _Comparative Morphology and Biology of the Fungi_, etc., 1887; Frank, _Die Krankheiten der Pflanzen_, 1895-96, and scattered in the works referred to in them and in the text. CHAPTER X. HEALTH AND DISEASE. _Variation--Disease--Comparison to a top. Health--Extinction of species--Natural demise. Examples of complex interactions in health--Interference, and tendencies to ill-health._