260 Inheritance in the Groundsel 



is a matter of difficulty, for the factors for hair and rays are by no 

 means the only ones present. 



There are at least two possible explanations of this behaviour, viz. 

 (1) that the four types of gamete RH, Rh, rH, rh are not produced in 

 equal numbers — gametic coupling ; and (2) that whether produced 

 in equal numbers or not, certain unions are preferred to others, e.g. 

 RH X RH and rh x ?•/(. to r-H x rH or Rh x Rh. This is not an im- 

 probable explanation, for certain combinations might easily have special 

 advantages ; e.g. more rapid growth of the pollen tube or quicker 

 response to the chemotactic stimulation of the ovule. 



Let us assume that R and H are really dominant, and simplify 

 Table X to 



TABLE XL 



No. of 

 plants 



4 Types 



Eip. Cross raised HR Ut hll hr 



Exp. 23 Lanuginosus x piaecox 83 Found 52 IG 9 6 



Katio 9:3:3:1 — Expected 4.5 15 15 5 



Ratio 22 : 5 : 5 : 4 — Expected 51 11 11 9 



The agreement with the latter ratio is remarkably close, and the 

 inference may fairly be drawn that coupling takes place according to 

 the system 2HR: IHr :lhR -.^hr. There is, however, more than this 

 to be deduced from the experiments. The excess of HHRR plants 

 suggests that the presence of R helps the factor H to assert itself more 

 effectively in the development of the hair character. 



The analysis of the F^ generation of these lanuginosus hybrids made 

 it desirable that further tests should be made by utilizing the F3 

 generation. The following cultures were therefore undertaken in 1911, 

 with seeds produced on selfed plants of 1910 (F^). 



The three objects already described were still in view — to secure 

 new hairy and non-radiate types, to test the general transmissibility of 

 hairiness, and to determine the relationships of the factors for hair and 

 rays — -but there was now a further one, viz. to test the accuracy of the 



