388 THE BIOLOGY OF FLOWERING PLANTS 



their stalks by their own weight, so that when the insect 

 visits another flower they are pressed against the stigma ; 

 this lies below the stamen, separated from it by the 

 beak or rostellum ; the rostellum also usually prevents the 

 pollinia from reaching the stigma of its own flower. 



Heterostyly. — The primroses are the classical examples 

 of the arrangement termed heterostyly. In Primula acaulis, 

 the primrose, and Primula veris, the cowslip, for example, 

 some individuals have flowers in which the five stamens 

 stand halfway down the throat of the corolla and a long 

 style carries the rounded stigma to the mouth. Other 



individuals, about one-half of the 

 whole population, have the stamens 

 at the mouth and a short style 

 bringing the stigma about halfway 

 up the tube (Fig. 59). As a re- 

 sult, the visiting insects, probably 

 moths, transfer pollen from the 

 long-styled, or " pin-eyed," flowers 

 to the stigmas of the short-styled, 

 or " thrum-eyed," flowers, and 

 vice versa. The larger pollen grains 

 of the thrum-eyed flowers also fit 

 better between the coarser stig- 

 matal papillae of the pin-eyed. 

 There is little chance of pollen of 

 one type of flower reaching a stigma of the same type, and 

 still less of self-pollination. In Ly thrum Salicaria there 

 are three types of flowers, long-, short-, and intermediate- 

 styled ; the first has short and intermediate stamens, the 

 second long and intermediate, the third short and long, 

 so that pollen is transferred from any one type to the 

 other two. 



Darwin (1876, 1877) showed that the " legitimate " 

 pollination of thrum-eyed stigma by pin-eyed pollen 

 habitually produces more seed and more vigorous ofl^- 

 spring than the " illegitimate " polUnation of thrum-eyed 

 stigma by thrum-eyed pollen, or of pin-eyed stigma by 



A B 



Fig. 59. — Primula veris ; 

 sections through, A, long- 

 styled, and B, short-styled 

 flowers. X 2. (After 

 Hildebrandt.) 



