tissues, which may he traced. hack etiologically to similar 

 causes often correspond with one another in their structural re- 

 lations. Therefore it is possible, without forcing matters, to 

 unite a, consideration of the etiology with that of the material 

 divided according to ontogenetic and histological characters. 



Moreover, most forms of pathological plant tissues and very 

 diverse ones may be traced back to factors other thah those a- 

 bovenamed. They are produced partly by injury, partly by infes- 

 tation with parasites, animal ol- plant. All abnormal tissues, 

 arising from "wound stimuli", will be designated as "callus for- 

 mations" in the widest sense of the word. We will have to speak 

 of callus-hsrper trophy, callus-hyperplasia and so forth, accord- 

 ing to the way in which the tissues are affected. On the other 

 hand, gall-formation will be spoken of, when parasitic organisms 

 of any kind whatever cause the formation of abnormal cells and 

 tissues. We v;ill report later in detail on gall -hypertrophy, 

 gall -hyperplasia and so forth. 



Although a descriptive treatment of pathological plant tis- 

 sues and a study of their development will especially deepen 

 and broaden our knowledge of the histological development of 

 the plants, yet we will be Obliged in considering the etiologi- 

 cal side to place above all physiological questions. We will 

 have to test the ways in which various factors affect the plant, 

 and the capacity for reaction of various cells and tissues to 

 certain stimuli; v/e v/ill be led from the study of formn to the 

 physiology of development especially of a pathological nature. 

 In this way also "pathological plant anatomy" will furnishccon- 

 tributions to the field, which we will follow Roux in designa- 

 ting as the developmental-mechanics of the organisms . 



It wiil be found that almost all pertinent questions are 

 easily subjected to experimental treatment. The phytopatholo- 

 gist is in the favorable position of being able to produce ex- 

 perimentally the abnormal tissues in v/hich he is interested. In 

 most cases., our knowledge of the factors at work does not ex-- 

 (7) tend beyond the very beginning; indeed there is such urgent need 

 of a thorough further analysis of these factors that the answer- 

 ing of most questions, and indeed the de??isive ones, must be 

 held over for future, researches. 



In each division we will follow the discussion of develop- 

 mental and histological character by rema^-ks upon etiology and 

 developmental-mechanics. In the concluding chapter, we will re- 

 capitulate briefly all that has been ascertained as yet concern- 

 ing the ways in which the different forces may act. 



The subsequent treatment of different abnormal tissue forms 

 will include a series of examples to illustrate each of these 

 forms and attention will be called to the pertinent literature. 

 Of course, it is not in our province to mention all the plants 

 capable of producing abnormal tissues, nor describe all the ab- 

 normal structures made known by personal investigg.tion or by 

 the statements of earlier authors. A multiplicity of examples 

 is to be avoided, such as could have been given easily for in- 

 stance in the. chapter on galls. Instead of describing exhaus- 

 tively all known cases which would be suitable in a manual, we 

 will sift the available material and limit ourselves to depict- 

 ing those cases important because of a wide-spread distribution, 

 or in any way interesting theoretically.. Moreover we have not 

 striven for a complete survey of all the literature concerned 

 v/ith abnormal cells or tissues. Articles v/ill not be named 

 which recapitulate only what is universally knov/n, or which re- 

 port new facts insufficiently. If, in spite of my endeavors to 

 reproduce all that is essential, important contributions have 

 escaped me, I ask for the forbearance of my reader, in view of 

 the wide range of literature here used. 



