157 



M94. Very pale and with many clumps. 



M95. Rapid-growing, pale, with few conidia and many clumps. 

 Several sectors later reverted to an appearance like that of the original. 

 Ml 01 Ml 05 were all fluffy, with white aerial mycelium. 



The discussion of the causal fungus of foot-rot has so far been based, 

 for simplicity, upon H. No. 1. Other strains of a Helminthosporium of 

 the same general type have been isolated from cases of foot-rot from Madi- 

 son county and have been proved capable of causing foot-rot. For ex- 

 ample, in the spring of 1920 five isolations from one lot of material were 

 made. These I designate as H. No. la, H. No. 16, H. No. Ic, H. No. Id, 

 and H. No. \e. All of these, in colony character and morphology, agree 

 closely with H. No. 1, but H. No. la, &, c, d, have graphs of conidial length 

 as shown in Fig. W, w r hile H. No. \e differed materially. It is obvious that 

 the first four may be considered as of one strain, the last, e, of another 

 strain, both strains differing somewhat from H. No. 1. The conidial 

 breadth of H. No. la was as follows: 



f M o- CV 



13 5.88 =*= .07 0.39 .05 6.79 .89 



Conidial septation of H. No. la, H. No. ib, and H. No. If, is given in 

 Figure X, Graphs 111-113. 



GENERAL DISCUSSION OF SALTATION 



The existing differences in definition and usage of the term mutation, 

 as also our very limited knowledge of cytological conditions in the genus 

 Helminthosporium and our ignorance as to whether it has sexual stages, 

 have led me to select the term saltation for the variations here discussed. 



The term mutant is defined by Dobell (43), following Wolf (127) and 

 Baur (11), as follows: "By mutation, accordingly, I mean a permanent 

 change however small it may be which takes place in a bacterium and 

 is then transmitted to subsequent generations. The word does not imply 

 anything concerning the magnitude of the change, its suddenness, or the 

 manner of its acquisition. The term denotes a change in genetic consti- 

 tution. All other changes which are impermanent depending generally 

 upon changes of the environment and not hereditarily fixed, are called 

 modifications. The word 'mutation' has been used with such different 

 meanings by so many bacteriologists and others, that the foregoing state- 

 ment seems called for." Brierley (28) defines a mutation as "a genotypic 

 change in a pure line"; and Vaughan (121), as "Those changes in form or 



