POLLENATION 



305 



FIG. 136. Polymorphic flowers of purple loosestrife 

 (Lythrum) 



In species having three forms of flowers the best seed- 

 production seems to result from the pollenation of a pistil 

 by pollen from a stamen of the corresponding length, 

 which must necessarily be from a different flower. (After 

 Darwin) 



may bend as it grows, dumping some of the pollen from 

 the anthers onto the stigma. In other cases the anthers are 



placed above the 

 stigma, so that the 

 pollen is brought to 

 the latter organ by 

 the action of gravity. 

 There are many 

 plants in which the 

 stigma regularly 

 pushes through the 

 ring of anthers and 

 thus becomes pol- 

 lenated. In other 

 plants this kind of 

 pollenation takes 

 place only under 

 special conditions, as in extreme dampness or extreme drought. 



358. Close pollenation and cross pollenation. Any process 

 that results in the transfer of pollen from the anther of a flower 

 to the stigma of the same flower is called close pollenation. 

 This designation is used to 



distinguish the process from 

 cross pollenation, in which 

 pollen is carried from the 

 anther of one flower to the 

 stigma of another flower (of 

 the same kind, however). 

 There are many plants in 

 which close pollenation is 

 impossible. 



359. Obstacles to close pollenation. There are three sets 

 of conditions in plants that interfere with close pollenation. 



I . Space relations. The relative position of stamens and pistils 

 within the flower may make close pollenation impossible. Or the 



FIG. 137. Stigma of a grass 



In wind-pollenated plants the stigmas usually 

 expose a large surface to the wind 



