POLLINATION AND FEETILIZATION 



153 



FIG. 137. Dichogamous flowers 

 of fireweed (Epilobium) 



At the left, earlier stage with 

 stamens mature but the stigmas 

 united into a club-shaped mass. 

 At the right, later stage with 

 stamens withering but the stig- 

 mas spread apart and ready for 

 pollination 



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 ; this is known as dichogamy. In some 



flowers, as in the fig wort and some 

 plantains (fig. 136), the pistils ma- 

 ture 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). 



Usually, as in some mallows 

 and in the gentians and fireweeds 

 (Epilobium') (fig. 137), the stamens 

 mature first. An insect visitor to 

 a newly opened flower in the stami- 

 nate condition becomes somewhat 

 covered with pollen. Then, flying to an older flower in the 

 pistillate condition, the insect is almost sure to leave pollen 

 on the stigmas and thus insure cross-pollination. 



It is common to find the stamens of a flower maturing a 

 few at a time, as in " nasturtium," buckwheat, and many other 

 flowers. This gives 

 more opportunities 

 for insects to carry 

 away the pollen 

 than would be pos- 

 sible if it all ma- 

 tured at once. 



145. Prevention 

 of self-pollination ; 

 dimorphism. A 

 means of prevent- 

 ing self-pollination 



which is even more effective than dichogamy is found in the 

 structure of flowers in which some have a long pistil and short 

 stamens, others a short pistil and long stamens. This condition 



FIG. 138. Lengthwise section of dimorphous 

 flower of bluets 



A, long-styled form ; B, short-styled form ; a, anthers ; 

 s, stigmas. About 3 times natural size 



