264 MISCELLANEOUS STUDIES 



In connection with the question of lethal action we must consider 

 the inheritance of doubleness of flowers. Snowflake seed regularly 

 gives a mixture of singles and doubles, about 53 per cent being doubles. 

 The doubles, which are totally sterile, are probably (Frost, 1915) pure 

 recessives (dd) for a single-double factor pair. The singles are always 

 heterozygous (Dd) ; crosses with pure single races (Saunders, 1911) 

 show that the approximately 1 : 1 ratio and the failure to produce pure 

 singles, with self pollination, are due to the fact that all the functional 

 pollen is doubleness-carrying (d). The excess of doubles over 50 per 

 cent has been explained by Miss Saunders (1911) as due to hetero- 

 zygosis of the singles for two linked complementary factors necessary 

 to singleness, and by the present writer (Frost, 1915) as due to lower 

 viability of the "single" gametes or embryos. The absence of func- 

 tional single-carrying pollen is apparently due to a lethal factor acting 

 after separation of the microspore tetrads, since the tetrads themselves 

 appear normal. 



In any consideration of factors linked with the single-double pair, 

 this semisterility of the pollen must be remembered. For example, 

 any dominant factor completely coupled with D in pollen formation 

 would be totally absent from the functional pollen, and the zygotes 

 produced by selfing would show directly the strength of linkage in the 

 ovules. 



The available data for the smooth-leaved type (table 24) are far 

 from constituting an adequate test of linkage, but they suggest that 

 the factors are independent. Certainly no high degree of linkage is 

 indicated by the totals, nor do the detailed data suggest that smooth- 

 leavedness is coupled with singleness in some parents and with double- 

 ness in others. 



We must admit that the peculiar inheritance of this type is not 

 yet positively explained. Evidently larger cultures are needed, and 

 crossing with the Snowflake type and with other commercial varieties ; 

 cytological study may also be required. Certain comparisons and 

 speculative possibilities deserve mention, however, especially since the 

 types yet to be discussed furnish additional evidence bearing on them. 

 We may compare the smooth-leaved and double types, as follows : 



DOUBLE SMOOTH-LEAVED 



1. A rare mutation of pure single 1. Apparently a common mutation 



("normal")- of pure Snowflake ("normal"). 



2. Recessive; extracted recessives 2. Apparently dominant; extracted 



are sterile mutant-type plants. recessives are fertile normal 



plants. 



[122] 



