Caroline Pellew and Eva Richardson Sansome 35 



grown in the past season, it has been found that the gametic ratio in the 

 ovules may be abnormal. For instance, from Aa x a, five plants as 

 mothers gave 17A : 50a, the missing A (coloured) plants not being 

 accounted for by any loss in the pods, the number of ovules that failed 

 to set seeds being only four. With the same pollen parents twelve 

 related (comparable) plants as mothers gave 69A : 70a. This result could 

 be explained by a redupUcation of A in the gametes giving coloured 

 plants. It is possible that cytological examination of this material will 

 disclose some abnormal behaviour to account for a reduplicated factor, 

 but whether chromosome association is found or not, it must be admitted 

 that the result of non-disjunction is not always gametic sterility. Should 

 chromosome association be foimd, the case would resemble that in 

 Datura of a fertile chromosome ring, which Blakeslee (1929) is inchned 

 to regard as brought about by regular disjunction. 



Summary. 



Crosses between a Thibetan variety of Pisum sativum and the variety 

 Duke of Albany gave fertile progeny except when a certain plant of the 

 Thibetan variety was used. In the exceptional case, half the gametes 

 fail to mature. The semi-sterihty is associated with the formation of a 

 ring of four chromosomes in the reduction divisions. 



The allelomorphs for round-wrinkled cotyledons (Rr) are connected 

 with ring formation and semi-steriUty in such a way that the Rr plants 

 are usually semi-sterile, RR and rr plants being fertile. In each class 

 there are approximately 1 in 8 exceptions. 



In certain fertile hybrids, the gametic ratio of lA : 3a was found. 

 The explanation suggested is, that the A factor is duplicated in the 

 gametes giving coloured (A) plants. 



PART II. 



In the offspring of crosses made between a particular plant of the 

 Une, "Thibet 7," of a Thibetan variety of Pisum sativum, and the variety 

 Duke of Albany, Miss Pellew observed the appearance of a regular 

 gametic sterility, resulting in the failure of about half the male and 

 female gametes. A cytological investigation of one of the semi-sterile 

 plants showed the presence of five bivalents and an association of four 

 chromosomes at metaphase of the first reduction division, instead of the 

 seven bivalents usually found in Pisum (Richardson, 1929). Further 



3-2 



