CROSS-FERTILIZATION OF PLANTS. 



265 



on the same plant. These do not open but shed the pollen upon the 

 stigma, in the mud, as we might say. These are called cleistogamous 

 flowers and I believe are borne only upon those plants which also produce 

 the ordinary showy flowers, thus insuring cross pollination at some period 

 in their history. 



Many flowers provide against self-pollination by some such common 



Fig 26. 



Fig. 27. 



device as in uie bluet (Houstonia), where some flowers bear the anthers 

 in the lower part of the cup and the stigma near the top, while in other 

 plants adjoining and perhaps raised from seed from the same plant, the 

 opposite arrangement of anthers and stigmas is found. One can readily 

 see how this will result in preventing self-pollination and furthering 

 cross-pollination. 



A still more effective method is found in many flowers where the 

 parts mature at different times, as has been already mentioned. The 

 most effective method, however, is shown in those plants like the willow 

 where only pistillate flowers grow on the one plant and only staminate 

 ones appear on another. These are called dioecious plants. The oak 

 represents the type where the two kinds of flowers are borne separately 

 on the same plant. These are called monoecious plants. 



