METHODS OF POLLINATION 



bluet (Housionia) and the bell flower (Campanula) are examples, 

 as well as the primrose so commonly grown in greenhouses. This 

 plant can be used for study and demonstration. Not only do 

 the length and position of the stamens and pistils in such flowers 



Fig. 133- 

 Dichogamous flowers of Primula. 



favor cross-pollination by insects, but Darwin has shown that in 

 some of them the pollen of the long stamens is impotent or weak 

 on the short pistil of the same flower, but is prepotent on the 

 long pistil of another flower, and so the pollen of the short stamens 

 is prepotent on the short pistil of a different flower. 



293. Flowers in which the stamens mature first. This 

 is shown in the great willow herb or fire weed (Epilobium). When 

 the stamens are mature the four stigmas are closed and the style 

 is bent backward. Later when the anthers have shed their pollen, 

 the style straightens out, and the stigmas open and become recep- 

 tive. In the evening primrose (CEnothera biennis = Onagra 

 biennis) the stamens also mature first (see paragraph 237). The 

 same is true of the high mallow with purple flowers. The sta- 

 mens are erect, shed their pollen, then wither and become recurved, 

 while the styles then elongate and the stigmas become receptive. 

 In most of the composite flowers the stamens mature first (see 

 the study of the sunflower, paragraphs 254-258). In the bell- 

 flower (Campanula) the stamens are joined by their anthers into 

 a tube, and mature their pollen before the stigma, which is inside 

 the tube, is receptive and open. The stigma now elongates, and 



