PREFACE TO THE ENGLISH EDITION vii 



the true status of agricultural chemistry, or of the study of 

 the physics and chemistry of soils, &c., to insist upon it that 

 no one can appreciate even the rudiments of plant-physiology 

 who does not make himself master of the facts of structure 

 and the essential phenomena of life by experimental investiga- 

 tion ; nor to point out that, as we come to know more about 

 the physiology and pathology of plants, we learn that the 

 chemistry of the soil is one of the least important factors we 

 are concerned with. 



These truths have to be faced, and in spite of the at first 

 sight depressing inference that the study of plant-pathology 

 and I suppose the same applies to animal-pathology de- 

 mands rigorous and active acquaintance with several other 

 branches of science. As I have stated in substance elsewhere, 

 we demand that the surgeon or doctor who attends us shall be 

 qualified properly to do his work, though we are perhaps not 

 always alive to the extravagance of our demands on his ability 

 and training ; and just as we cannot expect him to do his work 

 in diagnosing the disease or injury, and explaining and com- 

 bating or removing its cause, unless he is properly qualified, 

 by the requisite instruction in the physics, chemistry, structure, 

 and normal working (i.e. physiology) of the healthy body and 

 in the pathology of the case concerned, so can we as little 

 expect any one to deal with diseased conditions in plants who 

 is ignorant of their structure and physiology, and the patho- 

 logical conditions of the case concerned. There is, however, 

 the comforting assurance that the processes in plants, complex 

 as they are and we must not err in underrating this are 

 simpler than in animals, and must throw useful lights on all 

 general problems in Biology. 



The objection that there are epidemic diseases of plants 

 which we have as yet failed to prevent or overcome is obviously 

 no more valid than the cry that we cannot as yet stay the 

 progress of epidemics of influenza or cholera : the cases are 

 exactly parallel, and since we do not abandon or depreciate 

 the study of medicine because medical science is not yet in a 



