igi2] CURRENT LITERATURE 257 



and orderly presentation of most of the facts brought to light by extensive 

 cultures, and by means of which the results to be secured from any particular 

 mating is capable of prediction with a fair degree of accuracy. Double stocks 

 are totally sterile and must always be derived from singles, either self -fertilized 

 or crossed with other singles. Single stocks are of two kinds with respect to 

 their relation to doubleness, namely, " »0-rf-singles " which breed true to single- 

 ness when selfed or crossed with others of their own kind, and " (/-singles ' ' 

 which when similarly bred always produce both singles and doubles, the 

 doubles being generally in excess of the singles. Reciprocal crosses between 

 (/-singles and w(?-(/-singles give unlike results, owing apparently to a dif- 

 ference in the genotypic constitution of eggs and sperms in the (/-singles, 

 the eggs being of two sorts while the sperms are all equal. Doubleness is 

 recessive and disappears in the F I} but when the ^-single is the seed-parent 

 the Fj singles are of two sorts, some breeding true to singleness, others pro- 

 ducing both singles and doubles. When the (/-single is the pollen-parent, 

 the Fj singles are all of one kind and all give both singles and doubles in F 2 , 

 The ratios of singles to doubles in the pure-bred (/-single race have always 

 been suggestively near 7 : 9, thus indicating that the difference between singles 

 and doubles involves two genes instead of one. 



Miss Saunders assumes that singleness is due to the presence of two 

 factors X and F, the absence of one or both of these resulting in doubleness. 

 If these two factors were independent, the expected ratio would be 9 singles to 

 7 doubles, but if they are coupled according to the scheme discovered in sweet 

 peas, etc., n— 1:1:1: n— 1, the doubles would be in excess of the singles as 

 observed. The experimental results accord well with this assumption, and 

 make it probable that the coupling is of the form 15:1:1:15, though possibly 

 7-1*1:7. The peculiar feature in stocks, however, as compared with the 

 coupling in sweet peas, is the fact that neither X nor F are carried by the 

 pollen, and the coupling can show itself only in the constitution of the eggs. 

 The crosses with ^-(/-singles resulted in an unexpectedly small number of 

 doubles in F 2 , owing, as proved by further breeding, to the fact that in the 

 ^-(/-single race singleness is not produced by the joint action of X and F, 

 but of a similarly coupled pair X' and F\ To further complicate the situa- 

 tion, there occurs a " sulfur-white" (/-single race in which the plastid-color 

 is also " eversporting " in a manner quite similar to doubleness, the pure-bred 

 progenies always consisting of whites and creams as well as doubles and singles, 

 the singles being all white, the doubles mostly cream but also sometimes 

 white. The white plastids are assumed to be due to a factor W which is 

 borne by only a part of the eggs and by none of the sperms. Moreover W 

 is coupled with one of the factors for singleness, either X or F. Although in 

 the pure (/-single strains X, F, and W are carried only by the eggs, crosses 

 between (/-singles and no-d-singles of different plastid-color produced heterozy- 

 gous F x plants in which both pollen and eggs carried the coupled series of 

 genes. 



