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PLANT PHYSIOLOGY AND ECOLOGY 



triche, etc. Anemophilous flowers may lack stigmas, as in the 

 gymnosperms, or, more commonly, possess them, as in the angio- 

 sperms. Among the latter, five types of flowers are recognized 

 with respect to the way in which the pollen is exposed to the wind . 

 These are (1) the catkin-bearers, Salix, Populus, Betula, etc.; (2) 

 species with hanging flowers, Acer negundo, Rumex, etc.; (3) 

 flowers with long slender filaments, Plantago, Graminacem, Cypera- 



Fig. 33. An orchid, Calypso borealis, with one-flowered scapes, thus making 



xenogamy alone possible. 



ceoe, etc.; (4) flowers with explosive anthers, Urtica, Parietaria, 

 etc.; (5) species with fixed flowers, Typha, Potamogeton, Sparga- 

 nium, etc. 



148. Pollination by insects. Zoophilous species may be polli- 

 nated by birds, as in Bignonia, Impatiens, and Lonicera; by snails 

 as in Ariscema; or even by bats, as in the case of a few tropical 

 plants. Plants that are pollinated by insects are by far the most 



