838 ECOLOGY 



of our northern trees contrast with those of tropical trees in being predominantly 

 wind-pollinated. 



The advantages and disadvantages of wind pollination. The ques- 

 tion of advantage is here largely one of speculation. Undoubtedly a 

 great disadvantage in wind pollination is the enormous waste of pollen. 

 Probably not more than one out of a thousand or even out of many thou- 

 sand grains ever reaches the proper stigma. Perhaps, on the other hand, 

 the chance of a favorable wind is greater than that of a visit by the proper 

 insect. The dominance of wind pollination in such plants as the oaks, 

 pines, grasses, and sedges at once suggests that wind pollination certainly 

 is not detrimental. However, the great abundance of such plants 

 (especially the grasses and sedges) is quite as likely to be due to vegeta- 

 tive as to reproductive organs. 



Water pollination. Pollination through the agency of water is a relatively rare 

 occurrence but it is of much interest. In plants that are completely submersed 

 (as in several of the Potamogetonaceae and Najadaceae) the pollen grains are fila- 

 mentous structures that are as heavy as water or heavier, and the thick exine char- 

 acteristic of aerial pollen is-Jacking; such pollen grains upon release float below the 

 surface and may come into contact with the long exserted stigmas. 



In the tape gTass(Vallisneria) and in some of its relatives, pollination takes place 

 at the water surface. Vallisneria (fig. 1165) is a dioecious plant, whose pistillate 

 flowers are single and are borne on long scapes that bring the flower at the time of 

 stigmatic maturity just to the water level. The staminate inflorescences at maturity 

 become detached from their short scapes and rise to the surface; upon the opening 

 of the bract (spathe), the individual flowers also become detached and float about on 

 the water as miniature boats, the perianth opening and exposing the stamens. The 

 floating staminate flowers, like any small particles, swirl readily into the slight de- 

 pressions formed about the pistillate flowers, as about other objects on the water, 

 and come into contact with the stigma. After pollination the scape of the pistillate 

 flower coils up into a spiral, thus withdrawing the ovary below the surface, where the 

 fruit develops. In essential respects pollination in the water weed (Elodea) is com- 

 parable to that in Vallisneria. 



General characteristics of insect-pollinated flowers. Monodiny and 

 its advantages. Were it not so common, the symbiotic relation existing 

 between flowers and insects would be regarded as most marvelous. 

 From the standpoint of evolution, no great facts of nature are more 

 remarkable than that in many plant species the flowers remain unpolli- 

 nated unless they are visited by insects in search of nectar or pollen, 

 and that in a much greater number of species visiting insects are the chief 

 agents of pollination. Insect-pollinated flowers * are in great part mono- 

 1 Insect-pollinated flowers often are inaptly called entomophilous, that is, insect-loving. 



