PREFACE. 



It has often been represented to me that the 

 cultivators of plants, among whom are to be in- 

 cluded planters and foresters, as well as agricul- 

 turists and gardeners of every kind, are more 

 particularly concerned with, and interested in, 

 the maladies themselves of the plants they grow, 

 than in the life-history of the fungi, insects or 

 other organisms to which they are due, or in 

 the physiological processes which are involved ; 

 and although it is impossible to really under- 

 stand any disease unless we also understand the 

 processes by which it is brought about, there is 

 room for sympathy with the point of view of the 

 cultivator. He says, in effect, " I do not want to 

 know all about the biology of the fungus of wheat- 

 rust, or of the phylloxera, nor do I want to learn 

 what experts can tell me about the action of 

 bacteria in soil, or the process of starch-formation 

 in the leaves : I have neither the time nor the 

 means to master these details. What I want is 

 guidance as to what is wrong with my tomatoes, 

 apple trees, chrysanthemums, fir trees, turnips, etc., 



