PHYTOPATHOLOGY. 89 



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 symp- 

 toms {Diagnosis) on the one hand, and of causes 

 (^Aetiology) on the other, before any conclusions 

 of value can be drawn as to preventive or reme- 

 dial 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 im- 

 perfectly, 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 eg. drought, too high or too 

 low a temperature, light of insufficient or of 

 excessive intensity, a superfluity 6f water at the 

 roots, the presence in the tissues of parasitic fungi, 

 or that of worms or insects at the roots or else- 

 where, poisonous gases in the air, soil, etc., and so 



