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. 
M101—M105 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. 1a, H. No. 1b, H. No. ic, H. No. 1d, 
and H. No. le. All of these, in colony character and morphology, agree 
closely with H. No. 1, but H. No. 1a, 6, c, d, have graphs of conidial length 
as shown in Fig. W, while H. No. 1e 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. 1a was as follows: 
f M o CV 
13 5.88 + .07 0.39 + .05 6.79 + .89 
Conidial septation of H. No. 1a, H. No. 16, and H. No. tc, 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 
