FIELD CROPS. 537 



of leaf, are inherited independently. Hence the difHculty of producing a uni- 

 form strain after crossing will depend largely on the gametic condition of the 

 parents. If the parents differ in a large number of factors, the difficulties will 

 be much greater than if there are but a small number with which to deal. 



" The really important feature is that there is a segregation of quantitative 

 characters in the F- generation of tobacco crosses and that some segregates will 

 breed true in Fa. As this is the case, there seems to be no need of using a 

 different method when working with quantitative characters than for qualita- 

 tive or color characters. Since quality of cured leaf depends on many factors, 

 external as well as internal, it is probably unreasonable to expect a single external 

 character to be closely correlated with quality, but as homozygosis produces uni- 

 formity in both quantitative and qualitative characters it must tend to produce 

 uniform quality. The important matter in practice is simply to grow a sufficient 

 number of Fa and later generations to run a fair chance of testing out all the 

 combinations of factors possible to the parental varieties used." 



The results obtained in the Havana X Cuban cross " show clearly that an ex- 

 ternal similarity of size characters in tobacco varieties does not necessarily 

 mean a genetic similarity. . . . The general basis of the Mendelian conception 

 of heredity depends on the fact that the somatic appearance of a plant is not 

 a correct expression of its breeding nature. . . . That similar results are 

 obtained when dealing with size characters, and that in both quantitative and 

 qualitative characters it is impossible to know the germinal characters except 

 by a breeding test seems further proof of the belief that both are inherited in 

 a similar manner. The results of the sorting test of the parents and third 

 generation crosses show that heterozygosis affects quality and that uniformity 

 of external characters tends to produce uniformity of quality in the cured 

 leaves." 



" Our results show that the Fi generations of size crosses in tobacco are as 

 uniform as the parents and of an intermediate value; that there is an increase 

 of variability in F2 and where sufBcient variates are studied, a range of varia- 

 tion equal to the combined range of the parents ; that certain F2 individuals 

 breed true in F3, and that others give variabilities ranging in value from the 

 parents to that of the Fa generation. These results can be explained in essen- 

 tially the Mendelian manner — by the segregation of potential characters in the 

 germ cells and their chance recombination — therefore, from the plant breeding 

 standpoint there seems good reason for believing that quantitative characters 

 are inherited in the same manner as qualitative characters. 



" The production of fixed forms which contain certain desirable plant char- 

 acters is not, however, a simple problem, due to the large number of factors in 

 which plants of different races differ and because a superficial resemblance does 

 not necessarily mean a genetical resemblance. It is necessary to grow large F2 

 generations and to save seed from those plants which most nearly conform to 

 the desired type. Progeny of these F2 plants should be grown in row tests in 

 F3 and selection continued in later generations until the desired form has been 

 obtained. The length of time which it takes to produce a uniform type will de- 

 pend largely on the number of varieties which can be grown in F2 and the num- 

 ber of row tests which can be grown in Fa. 



" Quality of cured leaf is a complex character and due to many conditions, 

 envii'oruneutai as well as inherited. There is also the added difficulty tha* the 

 quality of leaf must conform to the trade ideals. The experiments here re- 

 ported indicate that a good quality of leaf can more generally be expected in a 

 hybrid, if the parents are both of high quality, than if one parent is a good 

 variety and the other somewhat lacking." 



A bibliography is appended. 



