APPENDIX 



As some readers may possibly care to repeat Mendel's 

 experiments for themselves, a few words on the methods 

 used in crossing may not be superfluous. The flower of 

 the pea with its standard, wings, and median keel is too 

 famiUar to need description. Like most flowers it is 

 hermaphrodite. Both male and female organs occur on 

 the same flower, and are covered by the keel. The an- 

 thers, ten in number, are arranged in a circle round the 

 pistil. As soon as they are ripe they burst and shed 

 their pollen on the style. The pollen tubes then pene- 

 trate the stigma, pass down the style, and eventually 

 reach the ovules in the lower part of the pistil. Fertilisa- 

 tion occurs here. Each ovule, which is reached by a 

 pollen tube, swells up and becomes a seed. At the same 

 time the fused carpels enclosing the ovules enlarge to 

 form the pod. When this, the normal mode of fertilisa- 

 tion, takes place, the flower is said to be selfed. 



In crossing, it is necessary to emasculate a flower on 

 the plant chosen to be the female parent. For this pur- 

 pose a young flower must be taken in which the anthers 

 have not yet burst. The keel is depressed, and the sta- 

 mens bearing the anthers are removed at their bas6 by a 



187 



