Presumptive Mutations in Wild & Cultivated Plants 57 



greater variation in the doubles than in the singles, and a high 

 negative correlation between petal number and stamen number in 

 F 3 plants, as would be expected if stamens are replaced by petals. 



The more complicated cases of doubling have been elucidated 

 by the well-known work of Miss Saunders on Petunia (1910, 1916a, 

 1916b), Matthiola (1911, 1913, 1915a, 1915b), and the wall flower 

 Chelranthus cheiri (1916c, 1917). According to Frost (1915), 

 doubleness is dominant in Petunia, and Miss Saunders (1916b) 

 concurs in this statement to the extent of finding that all true- 

 breeding singles of P. violacea, P. nyctaginiflora and various garden 

 strains, when pollinated from doubles give an F, containing both 

 singles and doubles. This indicates the heterozygous character of 

 the doubles, and hence the dominance of doubleness over singleness. 

 But since the doubles are sterile and P. violacea itself is a self- 

 sterile species it has not yet been possible to eliminate the possi- 

 bility that singleness is dominant, though Frost's hypothesis of 

 selective sterility accompanied by dominance of doubleness would 

 appear to fit the facts. In the wallflower, as in sweet william, 

 singleness is dominant. 



In the case of Stocks (Matthiola) the prolonged experiments of 

 Miss Saunders have gone far to clear up the hereditary relation- 

 ships. Singleness is dominant, and it is well-known that in the 

 various strains both pure-breeding and double-throwing (ever- 

 sporting) singles occur. The inheritance of singleness or doubleness 

 is independent of the pair of Mendelian characters for hoariness or 

 smoothness, and also of cell-sap colour, but is linked with plastid 

 colour (whether white or cream). All the singles derived from an 

 eversporting individual appear again to throw doubles. And in all 

 cases there is an excess of doubles in the offspring in the ratio of 

 something more than 7 singles : 9 doubles, from which it is 

 concluded that two factors are primarily concerned in producing a 

 single or a double. 



Moreover, breeding experiments show that all the functional 

 pollen grains of eversporting individuals carry doubleness, while 

 some of the ovules carry the factor for doubleness, and some that 

 for singleness, in the above proportion. Miss Saunders (1911) 

 explained the absence of doubles in the offspring of true-breeding 

 singles when crossed together, by an unequal distribution of the 

 factors among pollen grains and ovules. In the light of more recent 

 work it is probably simpler to explain the fact that some singles 

 breed true by assuming linkage of one determiner to a lethal 



