329 



Though the evidence for partial repulsion rests here upon 

 the single red, round ; plant which occured in family 61, it is in 

 reality much stronger than it appears, for the following reason. 

 All the plants in the above four families were hooded, i. e., 

 lacking in the factor for erect Standard (E), As we have already 

 pointed out 1 ) the three factors E. B. and L constitute a series 

 such that if any two are brought in to a zygote by different 

 gametes repulsion occurs between them. Until the present round 

 hooded red plant appeared we had never encountered this 

 combination in any of our experiments. It cannot therefore be 

 regarded as due to a stray seed from another family. And it is 

 evident that if the repulsion between any pair of these three 

 factors were complete, such a plant could never arise ; for in the 

 normal course the ebl gamete could never be formed. Only two 

 possibilities therefore are open. Either we must look upon this 

 plant as an unaccountable mutation, or we must consider that 

 the repulsion between B and L is partial. In the light of the 

 evidence affreded by the Cretin Sweet Pea we prefer the latter 

 hypothesis, and we are inclined to regard the partial repulsion 

 between B and L, as of the 1:7:7:1 type. On this hypothesis 

 we should expect one red round in every 256 plants (cf. Table II) 

 whereas experiment gave 1 in 419. At the same time we 

 recognize that the data are not yet sufficient to preclude the 

 1 : 15 : 15 : 1 System. It is worthy of note that the coupling 

 between B and L is usually on the 7:1:1:7 System, and it 

 would be interesting if in such cases as these the repulsion and 

 coupling System for a given pair of factors should prove to be 

 of the same intensity: In most cases this could not be tested 

 in practice owing to the very large number of plants required. 

 Thus the coupling between erect Standard and blue is on the 

 127 : 1 : 1 : 127 System, and if the repulsion were of similar 

 intensity we should expect only one hooded red in every 65, 536 

 plants. We may however State that in this particular case we 

 have grown over 4000 plants without meetmg with a hooded 

 red, so that the facts, as for as they go, point to a high inten- 

 sity of repulsion for factors exhibiting a high intensity of coupling. 

 It is obvious that the relation can only be worked ont where 

 the intensity of repulsion is low and it may be hoped that the 



i) Proc. Roy. Soc. 1 11, 6. 7. Ser. B. vol. 84. 



