172 MENDELISM 



keel is depressed, and the stamens bearing the 

 anthers are removed at their base by a pair of fine 

 forceps. It will probably be found necessary to tear 

 the keel slightly in order to do this. The pistil is 

 then covered up again with the keel, and the flower 

 is enclosed in a bag of waxed paper until the follow- 

 ing day. The stigma is then again exposed and 

 dusted with ripe pollen from a flower of the plant 

 selected as the male parent. This done, the keel is 

 replaced, and the flower again enclosed in its bag to 

 protect it from the possible attentions of insects until 

 it has set seed. The bag may be removed in about a 

 week after fertilisation. It is perhaps hardly necessary 

 to add that strict biological cleanliness must be 

 exercised during the fertilising operations. This is 

 readily attained by sterilising fingers and forceps 

 with a little strong spirit before each operation, there- 

 by ensuring the death of any foreign pollen grains 

 which may be present. 



The above method applies also to sweet peas, 

 with these slight modifications. As the anthers ripen 

 relatively sooner in this species, emasculation must 

 be performed at a rather earlier stage. It is generally 

 safe to choose a bud about three parts grown. The 

 interval between emasculation and fertilisation must 

 be rather longer. Two to three days is generally 

 sufficient. Further, the sweet pea is visited by the 

 leaf-cutter bee, Megachile, which, unlike the honey 

 bee, is able to depress the keel and gather pollen. 

 If the presence of this insect is suspected, it is 

 desirable to guard against the risk of admixture of 

 foreign pollen by selecting for pollinating purposes 

 a flower which has not quite opened. If the 



