4 BACTERIA IN RELATION TO PLANT DISEASES. 



this publication indefinitely since the writer no longer has living cultures of some of these 

 organisms. 



Troubles of other sorts have been encountered frequently and the reader would be 

 surprised, no doubt, to know how difficult it has been to obtain exact information on some 

 of these subjects. 



Some definite rule must govern the citation of literature when it is very abundant as in 

 case of some of these diseases, e. g., potato rots and pear-blight. Purely agricultural or 

 horticultural literature has not been cited in this monograph unless it has a direct bearing 

 on questions under consideration. Many speculative writings have been excluded. For 

 this reason literature on any given disease earlier than that definitely ascribing it to a 

 bacterial origin is, as a rule, not cited. Any other rule would have led to an endless number 

 of citations of very little worth. Perhaps not enough exclusions have been made. In this 

 matter it has been thought best to err on the side of fullness. Probably also some papers 

 have been overlooked since the literature is scattered through many languages. Occa- 

 sionally a citation has been made simply to show geographical distribution. 



As mentioned in vol. I the writer considers it advisable to state whenever possible the 

 exact temperatures at which experiments were made, but it happened frequently in the 

 great mass of notes from which the following pages have been compiled, that the expression 

 "room-temperature" is used. It may therefore be useful to certain readers to know that 

 the Washington room-temperatures, i. e., those of our laboratory, vary roughly as follows: 

 Summer temperature 25 to 35 C., occasionally 38 to 40 C. ; winter temperatures (heated 

 rooms) i8C.to 27 C., usually about 25 C. ; spring and autumn approximately 20 to 25C. 



While not averse to synthesis, the writer has usually followed the analytical method. 

 In general, "lumping" things not known positively to belong together is a worse proclivity 

 in natural history than excessive subdivision. Further experiments are often necessary, 

 and until such time it is best to keep separate subjects not demonstrated to be identical; 

 at least the writer has striven to follow this rule. The whole trend of modern scientific 

 research is toward analysis of phenomena, and only in the later stages of knowledge do 

 combinations come in properly to round out a subject throughly worked over. 



When one has to deal with many diseases some sort of nosology becomes necessary. 

 That which appears to be most convenient for the purposes of this treatise is, first of all, 

 the simple subdivision into three large groups: (i) the vascular diseases; (2) the parenchyma 

 diseases without hyperplasia; and (3) cankers, tubercles, and tumors in which there is a 

 more or less distinct hyperplasia. The reader should remember, however, that classifica- 

 tions are only conveniences. 



There is marked bacterial occlusion of the vessels in those diseases which I have 

 classed as vascular occlusions so extensive as to render this feature of the disease most 

 conspicuous, but it does not follow that there is not also some destruction of the paren- 

 chyma. Vascular bundles are not on the surface of the plant, and some preliminary bac- 

 terial destruction of the surrounding parenchyma must always occur before the disease 

 can take on its true vascular character, except perhaps in those comparatively rare cases 

 where the inoculation happens to be made directly into some bundle. Moreover, in later 

 stages of these vascular diseases bacterial pockets of greater or less extent are often formed 

 in the parenchyma, especially in its softer parts. The extent to which these closed cavities 

 occur varies greatly in different diseases. In the brown-rot of potato and tomato they are 

 numerous and often fuse into large tracts of disintegrated tissues (fig. i). In Stewart's 

 disease of sweet-corn, on the contrary, they are neither very large nor very numerous. 

 Exception, however, should be made of the inner husks where they are common. 



In the remaining groups of diseases, occurring with or without hyperplasia, it is not 

 uncommon to find occlusion of some of the vessels, although the first and principal dis- 

 turbance occurs in the parenchyma. As examples of this may be cited the basal stem-rot of 



