POLLINATION AND FERTILIZATION 



131 



123. Prevention of self-pollination, dichogamy. Of course 

 dioecious flowers like those of the willow cannot be self -polli- 

 nated. Monoecious ones like those of Indian corn (Figs. 335 

 and 336) are likely to be pollinated with pollen from another 

 plant. As regards bisexual flowers, it is evident that there 

 are many opportunities for self-pollination. But in all cases in 

 which cross-pollination produces more seed or stronger plants, 



or both, it is clear that any- 

 thing in the structure or mode 

 of development of the flower 



A 



FIG. 120. Dichogamous flowers of 

 plantain (Plantago lanceolata) 



A, earlier stage, pistil mature, stamens 

 not yet appearing outside the corolla ; 



B, later stage, pistil withered, stamens 

 mature. Six times natural size 



FIG. 121. Dichogamy in the 

 high mallow 



In A the stamens are mature but 

 the stigmas are pressed together 

 into a club-shaped mass (hidden 

 by the numerous stamens). In.B 

 the anthers are withered and the 

 stamens droop, while the stigmas 

 have separated and are ready for 

 pollination. After H. Miiller 



which tends to secure cross-pollination is highly advantageous. 

 One of the most effectual means of preventing self-pollination 

 in bisexual flowers is the maturing of the stamens at a different 

 time from the pistils, known as dichogamy. In some flowers, as 

 in the figwort and some plantains (Fig. 120), the pistils mature 

 first. In such cases the pollen from older flowers (in the stami- 

 nate condition) is transferred to the stigmas of recently opened 

 flowers (in the pistillate condition). Pollination of the plan- 

 tain shown in Fig. 120 is due to the wind. 



