CHAP. XI COUPLING AND REPULSION 119 



former contains a blue factor {B) which is lacking in 

 the latter {b). Sweet-peas exhibit an unusual feature 

 in plants, in that they produce one or other of two 

 distinct types of pollen. The majority produce 

 pollen which in the dry state is oblong in shape, 

 and generally spoken of as "long" pollen. In the 

 young state, and when moistened, it is oval and 

 contains 3 pores. In some sweet- peas, however, 

 the young pollen is round instead of oval, and con- 

 tains but 2 pores. The character of long pollen 

 (Z) is dominant to that of round (/). If a cross is 

 made between a long-pollened purple, such as that 

 illustrated on PI. IV., 4, and a round-pollened red 

 (PL IV., 7) the offspring are all long- pollened 

 purples. Nor does it matter which of the two is 

 used as the pollen parent. Such F^ plants, when 

 self-fertilised, give rise to an F.^ generation consist- 

 ing of the expected 4 types, viz. : purple long, purple 

 round, red long, and red round. 



So far the case runs along ordinary Mendelian 

 lines. Moreover, if we consider each of the two 

 factors concerned separately, the figures are in 

 accordance with expectation. The purples are to 

 the reds as 3 : i, and so also are the longs to the 

 rounds. It is only when we come to the relative 

 proportions in which these four classes appear in F^ 

 that a striking difference appears. Instead of the 

 four classes making their appearance in the normal 

 9:3:3:1 ratio, it is found that the two classes. 

 Purple long and Red round, are greatly in excess of 

 expectation. Nearly 7000 plants have now been 

 raised from this type of mating, and the actual 

 numbers of the four classes obtained are given in 



