18 THE COEFFICIENT OF MUTATION IN OENOTHERA BIENNIS L. 



last was a dwarf, which, however, has not flowered. Of the 

 remainder, ten individuals were pure biennis during their whole 

 life and in all their marks. They had the normal number of 

 chromosomes, namely 14, and gave a normal harvest of seeds. 

 The others, 8 in number, were different from these in almost all 

 respects, though but slightly. The color of their foliage was a 

 whitish green, the leaves more flat and with white veins. The 

 spikes were more elongated, the flower buds more slender, the 

 flowers small and erect, the fruits thin and cylindrical and rela- 

 tively poor in seeds. These plants had 15 chromosomes, like the 

 0. Lamarckiana lata studied recently by Gates and Miss Thomas. 1 ) 

 But they had none of the characters of a lata, showing thereby 

 that the number of chromosomes, even if differing from the type, 

 does not necessarily run parallel with the external features. 



Further studies will have to show why one-half of the progeny 

 of this cross came true to the characters of the pollen parent, while 

 the other half constituted a new and uniform type, differing from 

 all the mutations and hybrids hitherto studied in my experiment 

 garden; and expecially why the characters of the mother of the 

 cross should be wholly absent in its progeny. 



The first result of this state of affairs has been that the char- 

 acters which the semigigas mutants might show in early youth 

 remained unknown, and that it has not been possible to point 

 them out before the time of flowering. In July, all the spikes 

 were carefully mustered and four specimens of the semigigas type 

 were discovered. This makes a proportion of 4 to 8500, or about 

 0.05 per cent, showing the semigigas mutants to be only half as 

 frequent as the nanella. On later inspections no additional cases 

 were observed, and likewise intermediate or doubtful instances 

 were absent. The four plants were exactly alike, save that three 

 were very vigorous, and one, grown in a shady part of the garden, 

 was very weak. The chromosomes were counted in the first three 

 instances and found to be 21, as in the corresponding mutant of 

 Stomps. 



My four mutants were easily discovered by their broad conical 

 flower buds and their elongated spikes, which strongly contrasted 

 with the dense spikes of the surrounding biennis. They reached 

 the same height as these, the lowest flower being 90 cm. above the 



i) Gates, R. R., and Thomas, N., A cytological study of Oenothera 

 mut. lata and O. mut. semi lata in relation to mutation. Quat. Jour. 

 Micr. Sci. 59:523, 19 14. 



