GEORGE K. K. LINK 595 



in part so imperfectly differentiated culturally that host specificity has been used 

 as the final diagnostic character, have demonstrated that in the main the method is 

 suited to differentiate species which are not readily distinguishable culturally, but 

 which show some host or symptom specificity. Lacey,' and Link and Taliaferro^ showed 

 that the same situation holds for certain organisms which cause soft rot of vegetables. 

 In the light of these data, one is justified in venturing the statement that in the 

 main the species which have been created on the basis of even slight difference in cul- 

 tural behavior and appearance, especially when these are correlated with host and 

 symptom specificity, are probably deserving of species rank. Lacey and Sharp's de- 

 tailed cultural work with soft rot and bean pathogens also indicates that most sero- 

 logical differences will be found to be correlated with morphological and cultural dif- 

 ferences, however slight. 



COLONY INSTABILITY OR VARIATION 



■ The concept of normal culture, colony, or cell, has dominated phytobacteriologi- 

 cal researches, and consequently colony variations have received relatively slight at- 

 tention save mention as disconcerting phenomena, or have been discarded as degen- 

 eration or involution forms, or even as contaminations. Recently, Sharp-' described 

 variant colonies of Bad. phaseoU sojense which became umbonate and wrinkled and con- 

 voluted in five to eight days. He found, on the one hand, a correlation of the rough (R) 

 colony form with other properties such as high agglutinability, lesser virulence, and 

 lesser or no motility, and, on the other, correlation of the smooth (S) form of colony, 

 which is the normal colony of the species, with low agglutinability, greater virulence, 

 and greater motility. This study has brought out the striking parallelism between 

 bacteria being studied in the animal and plant fields. ^ Link and Hull^ have reported 

 other plant pathogens which produce smooth and rough colonies. These are Bad. 

 citri, a strain of Bad. tumefaciens which gives rise to colonies that are partly rough and 

 smooth at first, or entirely rough, but which speedily become smooth, and a strain of 

 Bad. medicaginis var. phaseolicola, No. 23, which makes a rough-and-smooth growth 

 on agar. 



Sharp's findings suggest that it will be worth while to devote considerable atten- 

 tion to aberrant colonies. In view of the fact that relatively few investigators de- 

 scribe or illustrate the aberrant colonies, it is impossible to draw any conclusions from 

 the literature as to how generally the phenomenon of rough and smooth colonies oc- 

 curs among plant pathogens. Fortunately, however, under the leadership of Smith 

 enough investigators have described and illustrated not only the normal but also the 

 variant colonies to enable one to infer that smoothness and roughness and kindred 

 phenomena have been observed before. 



Smith'' has described for Bad. tumefaciens colonies with smooth, shiny surfaces as 



'Lacey, Margaret, S.: "Studies in Bacteriosis," XIII. Ann. Appl. Biol., 13, i-ii. 1926. 



^Link, George K. K., and Taliaferro, W. H.: ibid., 85. 1Q2S. 3 Sharp, C. G.: Inc. cit. 



-•[For a review of tlie earlier studies on (R) and (S) colony forms, cf. chaps, xlii and vii in this 

 volume. — ^Editors.] 



5 Link, George K. K., and Hull, K. : "Smoothness and Rouglmess, and Spontaneous Agglutination 

 of Bad. citri, Bad. medicaginis var. phaseolicola. Bad. phaseoli sojense (Rj, and Bact. tumefaciens,'' 

 Zio/. Gas., 83, 412-19. 1927. 



^ Smith, E. F.: Bacterial Plant Diseises. 



