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 super- 

 fluous. The flower of the pea with its standard, 

 wings, and median keel is too familiar to need 

 description. Like most flowers it is hermaphrodite. 

 T3oth male and female organs occur on the same 

 flower, and are covered by the keel. The anthers, 

 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 

 penetrate the stigma, pass down the style, and 

 eventually reach the ovules in the lower part of the 

 pistil. Fertilisation 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 fertilisation, 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 purpose a young flower must be taken 

 in which the anthers have not yet burst. The 



213 



