PREFACE TO THE GERMAN EDITION. 



For the third edition of my manual I have requested the assistance of 

 Professor Dr. Lindau and Dr. Reh. In the second volume of the work, the 

 former has treated of vegetable parasites and in the third volume the latter, 

 the animal enemies of plants. 



Such help seemed necessary because, since the appearance of the second 

 edition, the published results of investigations have been so numerous that 

 too long a time would have been required for mastering the material. Other- 

 wise when the last sheets appeared the first would have become obsolete. 

 Even with this division of the work, this unfortunate condition has not been 

 entirely overcome and an attempt has been made to obviate the difficulty by 

 listing some of the more important recent material in a supplementary biblio- 

 graphy. If the absence of some works, especially of the earlier literature, is 

 noted the explanation lies in the fact that we have emphasized especially 

 those studies necessary for the support of our presentation of the subject. 

 A more detailed bibliography would be possible only if the individual diseases 

 were treated in monographs. 



I kept for my own work the revision of the first volume, comprising the 

 non-parasitic diseases. The fact that this volume is the most extensive is ex- 

 plained by my standpoint, already sufficiently characterized in the preface to 

 the second edition,— because I lay the chief weight on a knowledge of the 

 diseases produced by atmospheric, soil and cultural conditions. The distur- 

 bances caused by these factors are not only the most abimdant and perma- 

 nent but also often form the starting point for parasitic diseases. 



On this account, supported by my own studies and the observations of 

 other investigators, I was especially anxious to show how the same plant 

 species could be changed structurally and in habits of growth according to 

 position and the constitution of the soil. Individuals are sometimes more 

 disposed to a definite form of disease or are more resistant to it, according 

 to the difference in their constitutions. 



This holds good also for their behavior towards parasitic organisms. It 

 is thus evident that not only must the latter be combatted by directly destruc- 

 tive methods but also the chief emphasis should be laid on the possible con- 

 stitutional change of the host plant. Therefore, we will find the most essen- 

 tial task to be the breeding of resistant varieties. At the time the first 

 edition of this work was published, the undersigned stood alone as represen- 

 tative of this theory of predisposition to parasitic attack, but now many of 

 the most prominent investigators are counted among its supporters. 



And thus I hope that the idea for which I have fought since the be- 

 ginning of my scientific activity, that is, the formation of a rational plant 



