20 THE COEFFICIENT OF MUTATION IN OENOTHERA BIENNIS L. 



able to carry out the experiment. It may be reduced very essen- 

 tially by a previous study of the marks of the young rosettes of 

 0. biennis semigigas, so as to be able to plant out almost only these, 

 hoping to find the gigas among them; or by studying the external 

 influences which may increase the degree of mutability of the 

 parents in the desired direction. 



Sulfurea mutants have been far less rare. This was to be expected 

 from the fact that Stomps had 4 of them among 920 plants. 

 From the parent type they differ only in the color of their petals, 

 which is a very pale yellow. It is so pale that collectors, who see 

 the variety in our sand dunes, often call the petals white. In the 

 cultures they are easily seen as soon as the flowers open, especially 

 in the evening. I found 27 of them among my 8500 plants, making 

 a percentage of 0.3 per cent. They occurred in the progeny of all 

 the 7 parents of my stock, 13 in the third and 14 in the fourth 

 generation. There were 6 parents, whose progeny contained 

 0. 1 -0.3 per cent, and one with 0.7 per cent (of the fourth generation). 

 It is possible that this last parent had been more favored by 

 external conditions than the three others of the same group, 

 although it grew among them and did not show any higher degree 

 of vigor. 



The fact that sulfurea mutants were observed in the progeny 

 of every one of the 7 parents of my culture directly proves this 

 line of mutability to be hereditary in the whole family derived from 

 the 1905 rosette from Wyk aan Zee. In combination with the 

 sporadic occurrence of the pale-colored variety in our sand dunes, 

 we may further infer that this mutability is hereditary in the whole 

 stock of our country, and probably also in the whole species, since 

 sulfurea plants have been found from the time of Tournefort in 

 France and other European countries. 



From the mutants constant races may be derived. I sowed the 

 self-pollinated seeds of the four mutants of Stomps and culti- 

 vated 205, 225, 271 and 358 seedlings, altogether 1059 plants, all 

 of which have flowered and produced only pale-yellow petals, 

 making a very striking impression of constancy. 1 ) When crossed 

 with the pure species, the sulfurea strains give uniform hybrids 

 which are patroclinous. Those of 0. biennis x sulfurea have the pale 



i) Self-pollinated seeds of this second generation of O. biennis sulfured 

 are available for exchange in return for other races of mutating primroses. 



