182 



GENETICS IN RELATION TO AGRICULTURE 



two varieties of tobacco of the species Nicotiana longiflora. The smaller 

 of these two varieties has a tube length of about 40 mm., whereas the 

 contrasted variety bears flowers the tube length of which is over twice 

 as great, namely about 93 mm. The two varieties had been self-fertil- 

 ized for a number of generations preceding hybridization, and since it 

 can be demonstrated that continuous self-fertilization tends to reduce a 

 variety to a homozygous condition, it is fair to conclude that the parents 

 represented varieties homozygous for nearly, if not quite, all their fac- 

 tors. We are not surprised, there- 

 fore, to find that they display only 

 a slight variability in flower size. 

 This slight variability is to be con- 

 sidered merely an evidence of the 

 influence of external conditions and 

 of inherent variability in character 

 expression and not of internal 

 heterozygosity, for there is a limit 

 below which it is apparently im- 

 possible to force the reduction in 

 variability of any given character. 

 In this case the accompanying table 

 which has been reproduced from 

 East in its entirety will serve as the 

 material for the following discussion. 

 It will be seen in Table XXXIII 

 that when the two varieties were 

 crossed the FI distribution occupied 

 a position midway between the two 

 parents. The number of plants 

 grown was somewhat larger than that 

 for the parents, consequently the 

 range covered by the FI distribution is slightly greater, but calcula- 

 tions of the coefficient of variability show that the variability of 

 the FI is only slightly and not significantly greater than that of 

 the smaller flowered parent. When we look at the F 2 from such a 

 cross, we find that although it, like the FI, occupies an intermediate 

 position, the range has been doubled and this in spite of the fact that the 

 population contained only a few more individuals than that of the F\. 

 This increased variability is borne out by calculations of the coefficients of 

 variability which are over twice as great for F 2 as for FI. That the in- 

 creased variability in F? is the result of genetic segregation of some sort 

 is shown by the distributions of F 3 families. They are strikingly different 

 from each other in their position on the range, and in the variability which 

 they display, as is shown clearly in the table. 



FIG. 87. Average flowers of two 

 varieties of Nicotiana lortgiflora with an 

 average flower of the Ft from a cross be- 

 tween them in the middle. (After East.) 



