INTEODUCTION. 



Relative and Absolute Diseases. — Plants, like man and animals, 

 in virtue of their living cellular constitution, are liable to disturb- 

 ances of their physiological equilibrium, that is, to diseases. The 

 diseases of plants are quite as well defined as those of animals, and a 

 certain analogy exists between them, so also with the inducing causes. 

 We again find amongst these causes the same physical, chemical, or para- 

 sitic elements. These causes generally co-operate to produce disease ; 

 but one cause is always preponderant and serves as a basis for clas- 

 sification. Plant diseases are divided into (a) relative and [b) absolute 

 diseases. Eelative diseases are a return to the natural wild state of 

 the plants artificially selected and profoundly modified by man, and so 

 it is that Brussels sprouts, turnip cabbage, and the difterent species of 

 cauliflower are derived from a species of cabbage to which a disease 

 Parenchymatosis has been imparled artificially by over-nitrogenous 

 feeding systematically pursued for several generations. The tend- 

 ency of these plants to form large fleshy buttons, in the form of a 

 cabbage-head, or excrescence of the stem, is not at all inherent in the 

 species, and it is not unusual in dry seasons to observe a leturn of 

 these plants to the primitive condition. This degeneration is regarded 

 as a relative disease. It is the same with fruit trees ; we skilfully 

 maintain by pruning the morbid condition which forces them to pro- 

 duce an exaggerated amount of fruit so as to accomplish in this way 

 the end which we assigned to them. Their return to the natural state is 

 regarded as a degeneration or a relative disease. Absolute diseases 

 result, on the other hand, from more or less profound alteration,, 

 general or local, of the organs of the plant ; from a more or less ex- 

 tensive and complete alteration of the cellular tissue. It is to absolute 

 diseases alone that the etiological observations which are about to be 

 examined apply. 



Etiology. — Etiology is that part of pathology which is con- 

 cerned with the research of the origin of diseases, and examines the 

 causes which induce them. The latter are divided into effective 

 causes (those which actually cause the disease) and adjuvant (helping, 

 predisposing) causes (those which place the plant organism in such a 

 condition that the efl'ective causes can act). In practice, these causes 

 are confused and linked together in such a way that an adjuvant cause 

 in a given disease may become an effective cause in another disease. 

 It has been seen that the etiological factors of disease may be divided 

 into (1) physical, (2) chemical, and (3) parasitic. 



(i) Physical Causes. — These depend on climate and season. 

 Heat, cold, drought, and humidity, more or less sun-light, are factors 



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