1911.] A Case of Gametic Coupling in Pisum. 



11 



There is no difficulty in distinguishing the tendrilled from the acacia 

 plants when six to seven leaves are developed, but in some of the tendrilled — 

 doubtless the heterozygotes — the apical tendril is sometimes strap-shaped, 

 especially in the youngest tendril-bearing leaves. 



On one occasion at Verrieres, Acacias came in a strain of Sandar's Marrow. 

 Though natural cross-fertilisation is extremely rare among peas (much rarer 

 than in sweet peas) we can hardly doubt that these Acacias were recessives 

 extracted after an accidental cross with the pure strain growing in the same 

 garden. Both among various peas grown at Verrieres, at Beading, and at 

 Grantchester, a few unquestionable examples of crossing have been observed 

 since critical attention has been devoted to the study of heredity in peas. 

 The crossing is probably effected by visits of Megachile to flowers in which 

 for some reason their own pollen has been inoperative. 



In the case of the derivatives from the Sandar's Marrow strain the 

 occurrence of strap-shaped tendrils, presumably on the heterozygotes, has 

 been often observed, and some plants have many such intermediate tendrils. 



The original cross which gave coupling between T and B was in the 

 form TB x tr. Experiments are now in progress for testing whether when 

 a cross is made in the form Tr x tB the gametogenesis of Fi will show 

 repulsion of T from B, and on the analogy of what has been seen in sweet 

 pea and in Primula sinensis this result may be confidently anticipated. 



Whether any similar inter-relation exists between the tendril factor and 

 factors other than that for round seed cannot be yet stated, but it is 

 practically certain that the factors for yellow seed and for tall stem do not 

 stand in any such special relation to it. The case is also interesting 

 inasmuch as it is the first yet met with in which neither of the coupled 

 factors is in any way concerned in determining pigmentation. 



In conclusion it may be remarked that an identical " acacia " variety 

 exists in the sweet pea, and its properties are also under investigation. In 

 the sweet pea, however, there is no variety with truly wrinkled seed. The 

 types with self-coloured lavender flowers have somewhat shrivelled seeds, 

 but the starch of these is normal. 



