CHAPTER LVIII 



POLLENATION 



356. Function of poUenation. We have learned that flowers 

 are seed-producing structures, and that seed production takes 

 place only after fertilization. But in seed plants (most of which 



are land plants) 

 the parts of the 

 organism which 

 bear gametes 

 are so situated 

 that fertiliza- 

 tion is possible 

 only after pol- 

 lenation ; that 

 is, the transfer 

 of pollen from 

 the anthers to 

 the stigma. In 

 these plants re- 

 production de- 

 pends in a 

 rather peculiar 

 way upon cer- 

 tain external 

 factors. 

 357. Self-pollenation. In many plants the transfer of pollen 

 is brought about by the growth movements of the parts of the 

 flower. The style, in elongating, may bring the stigma into 

 contact with the anthers ; a movement of the corolla may 

 push the stamen against the stigma ; the stalk of the flower 



304 



Fig. 135. Dimorphic flowers of Chinese primrose 



{Primula) 



In these plants there are two forms of flowers (hence the term 

 dmiorphk). In form A the anthers are high and the stigma is 

 low ; in form B the anthers are low and the stigma i^ high. The 

 pollen from form A is prepotent for the pistil of form B, and 

 vice versa. That is, long-stamen pollen must reach long-style 

 pistil, and short-stamen pollen must reach short-style pistil, to 

 produce the best or the most seeds. This necessitates cross 

 pollenation, or at any rate handicaps close pollenation 



