32 Genetical and Cytological Studies on Pisum 



Nineteen F^ families from the crosses 230 and 233 were raised in 

 1929, eight from half-steriles and eleven from fertiles. In this season the 

 condition of the plants was exceptionally good, in spite of a prolonged 

 drought. Germination on the whole was excellent, and a complete record 

 of almost every plant was made. All the plants derived from fertile 

 parents were fertile, without exception. The famihes raised from half- 

 sterile plants (Table II) are of the same constitution in respect to Rr 

 and the incidence of sterihty as in the F^, and in view of the complications 

 involved, the agreement between the different families is remarkable. 

 It was in this season that a cytological inquiry for the cause of the 

 sterility was begun by Miss Richardson (Mrs Sansome), and ring formation 

 was discovered. Mrs Sansome's investigation was first restricted to the 

 family 256/29, and in this family extensive counts of seeds and of aborted 

 ovules were made by her, assisted by Miss Bolt, in order to find the exact 

 degree of gametic sterility. The counts are summarised in Table III. 



TABLE III. 



Numier of aborted ovules, in perceiUages, of forty-three half-sterile plants in F^ 

 family 256/29. Average number of ovules per plant {rudiments and fertile) 147 ; 

 tAe smallest number included, 37; the largest, 266; six plants below 100 ovules. 



Of the forty-three plants examined, thirty-four have over 50 per cent, of 

 the ovules aborted, and sixteen have over 55 per cent, (up to 62 per 

 cent.). The deviation from 50 per cent, may well have a significance in 

 relation to the chromosome ring, but it must be noted that even the most 

 fertile plants may fail to set seed in the later stages of growth, and that 

 some at least of such failures would be classed with the aborted ovules. 

 A number of small counts have also been made in other families, from 

 the low pods of half-sterile plants, and they show that any departure 

 from 50 per cent, gametic sterility is rare. Comparatively few pollen 

 counts have been made; they are, however, in general agreement with 

 the ovule counts. The few exceptional plants were found too late in the 

 season to allow of cytological examination. 



A general explanation of the genetical facts I have described, and 

 also of the cytological observations (see Mrs Sansome's report) is given 

 by the theory of segmental interchange between non-homologous 

 chromosomes. It appears probable, however, that some modifications of 



