the flowers up out of the water. For our species this is impos- 

 sible except where subsiding water level has brought the vegeta- 

 tive body developed in deeper water to lie near the surface 

 through subsidence. 



The staminate flowers of Elodea ioensis (6) and the pistillate 

 flowers of Yallisneria spiralis are brought to the surface from 

 considerable depths through the elongation of the pedicel or in- 

 dividual flower stalk. This door is closed to Heteranthera dubia 

 as its flowers are sessile on a usually deeply submersed stem. 

 It can not employ the principle of detachment in any event asi 

 its flowers are perfect ; the staminate flowers of Elodea canadensis 

 (7) and Yallisneria spiralis may detach and come to the surface 

 to pollinate the anchored pistillate flowers. 



Another way to escape close pollination would be to elongate 

 the flower itself, such as seen in the pistillate flower of Elodea 

 canadensis (7) and E. ioensis (6). If there has been any spe- 

 cialization of Heteranthera duhia it has been along this line for 

 the tubular perianth and elongated style operate to push the sta- 

 mens and stigmas out of the water if only slightly submerged. 

 Since this same form of flower occurs in the terrestrial species 

 of this genus it is evidently an ancient rather than a recent spe- 

 cialization. The slightly longer perianth tube in this species may 

 be correlated with the reduced illumination since approximately 

 one-half of the light is lost at the surface of the water. The 

 total achievement, however, is utterly inadequate to the needs 

 for the flowers can reach the air only when attached at the most 

 an inch below the surface. 



The remaining alternative, that of subsurface pollination 

 through the transfer of pollen grains under water, such as is 

 employed by Ceratophyllum demersiim is impossible for Heter- 

 anthera dubia as it has perfect flowers that only open at the sur- 

 face. It is thus cut off from what would seem to be the most 

 desirable method for plants of its habitat, — a iyp^ of cross 

 pollination which allows plants to grow and set seeds at as great 

 depth as light pennits. 



From the above it is seen that all the possibilities of cross 

 pollination except that achieved through flower elongation are 

 closed, or have not been developed at least, so that if seeds are 

 to be set with certainty and in considerable numbers it must be 

 through close pollination under water excepting the relatively 



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