1919] Frost: Mutation in Matthiola 125 



It should, however, first he noted that, as will appear later, phe- 

 nomena of apparent linkage in the case of certain other types (crenate, 

 slender, and narrow) suggest that these forms commonly arise from 

 Snowflake by segregation rather than by immediate mutation. The 

 obvious objection to this hypothesis is the fact that the apparently 

 mutant types seem to be dominant to the "normal" or Snowflake type. 

 This objection can be met by assuming the presence of dominant in- 

 hibiting factors in the Snow-flake parents that give apparent mutants.^' 



If the apparent mutants of the smooth-leaved type are thus pro- 

 duced by crossing over in a set of balanced factors, the lethal ' ' balanc- 

 ing" the smooth-leaved factor itself may be distinct from that which 

 sterilizes the singleness-carrying pollen. In considering the results 

 here reported, therefore, we must always bear in mind the possible 

 presence of several unidentified lethal factors. If the apparent absence 

 of linkage between the smooth and double factors is not misleading, we 

 must suppose that these factors are carried by different pairs of 

 chromosomes; considerations advanced by Muller (1918, pp. 479-482), 

 however, make it rather probable that the commoner t.ypes of apparent 

 mutants here discussed are all due to factors carried by one pair of 

 chromosomes, the pair containing the factor for doubleness and its 

 normal allelomorph. 



3. THE LAEGE-LEAVED TYPE 



A double of this type probably occurred in the 1907 cultures, 

 though its appearance attracted so little attention that no record was 

 made. In the field cultures of 1911 (table 3) several individuals sug- 

 gested a gigas type, though there seemed to be intergradation with 

 Snowflake. In the 1912 cultures a single with leaves "long, rather 

 narrow, thick" developed normally and produced an abundance of 

 good seed; from this individual (28a) all cultures of this tj^pe are 

 descended. 



This type is stout and coarse throughout, and late to flower. The 

 leaves are strikingly long, thick, and rigid, though as a rule relatively 



15 A letter suggesting this explanation was received from Dr. Muller soon 

 after the same idea had been outlined in the ' ' General Discussion ' ' section below. 

 Dr. Muller kindly gave further attention to difficulties at first encountered by 

 the present writer, materially assisting in the formulation of an apparently 

 tenable form of the hypothesis. Since, however, this scheme may seem "far- 

 fetched ' ' and unduly complex, it appears desirable to leave the original discus- 

 sion of the individual types substantially unchanged. When the difficulties 

 encountered by the assumption of frequent true mutation have been more fully 

 presented, the need for some such addition to the scheme will be more evident. 



