510 ON THE DISTRIBUTION OF MUTANT CHARACTERS AMONG 



single group. Evidently there are some main mutations, from which 

 the others may be derived, or to which they can be subordinated. 

 These primary mutations have been described by one of us under 

 the name of dimorphic mutants (de Vries 1916), since, after self- 

 fertilization, their progeny mainly consists for one part of indivi- 

 duals of the parental type, and for the remainder, of specimens of 

 the type of the original species, Oe. Lamarckiana. All of them are 

 heterogamous, since their characters are not reproduced by means 

 of their pollen. 



Of these dimorphic mutants there are six main types. Three are 

 old ones, having been observed since the beginning of the experi- 

 mental cultures of Oe. Lamarckiana (de Vries 1901, pp. 273, 288, 

 300). They are called mut. lata, mut. scintillans and mut. spathulata. 

 The three others have been described for the first time in 1916, in 

 the article already referred to. Their names are Oe. cana, Oe. liquida 

 and Oe. pallescens. 



Among these six forms two are known to produce almost annually 

 and in a high percentage another definite type, which, however, 

 is distinct in the two cases. Mut. lata produces about 2 percent 

 albida or more and mut. scintillans throws off oblonga in a still larger 

 percentage. In some instances it reached 6 percent of the whole pro- 

 geny (de Vries 1913, p. 314). The reverse process does not occur, and 

 therefore albida must be considered to be secondary to lata, and 

 oblonga to be derived from scintillans. 



Albida and oblonga are, however, not dimorphic in the sense given 

 above. They constitute constant races, but this constancy is due 

 in part to the presence of barren grains among their seeds. They 

 have two kinds of female gametes but only one kind of these is repre- 

 sented among their male sexual cells. From this very distinct charac- 

 ter, they may be called "one-and-one-half" or "sesquiplex" mutants. 



In considering the remainder of the 15-chromosome mutants it 

 is to be noted that one ot them, Oe. candicans, is also a sesquiplex 

 type, but closely related to Oe. cana, whereas almost all the others 

 are to be derived either from Oe. lata or from Oe. scintillans. 



Semi-gigas types are known to have 21 chromosomes, and their 

 derivatives vary in this respect with numbers going from 15 to 20. 

 Our race of Oe. Lamarckiana gigas has 28 of such rods, as has been 

 discovered by Miss Lutz and Gates. These special types, however, 

 must be excluded from the present discussion since in them the 

 changes ar not limited to a single chromosome, but include, at 

 least in their origin, all of them at the same time. 



