190 



FRESH-WATER BIOLOGY 



Vallisneria spiralis. 



short exposure to the air the flowers reflex the sepals to form a 

 Uttle boat which floats about with the dehiscing stamens exposed 

 to the air, so that whenever the boat lodges by a pistillate flower 



some pollen is deposited upon the 

 receptive stigma. The pistillate 

 flower is solitary upon a long stalk, 

 which, rising from the leaf axils, 

 elongates very rapidly until the 

 flower floats on the water surface, 

 when the stigma is soon exposed 

 to receive the pollen from the 

 passing boats of staminate flowers 

 (Fig. 254, page 181, and Fig. 266). 

 Sometimes where Vallisneria is abundant the water surface is 

 completely covered by the staminate flowers, just as Lemna, the 

 duckweed, often covers certain areas. As soon as the pistillate 

 flower is fertilized the stalk contracts to a spiral, thus drawing the 

 flower under water to mature the fruit. 



To what extent Vallisneria is propagated by seed is not known. 

 It has been necessary for the writer to take hundreds of these 

 plants from the lake for experimental pur- 

 poses and a seedling has not as yet been 

 found. The plants growing in water 2.5 to 

 3.5 meters deep frequently do not flower at 

 all but readily propagate by runners. 



As previously mentioned, Zannichellia 

 palustris conducts its pollination under 

 water (Fig. 267). The staminate and pis- 

 tillate flowers stand in the same axil 

 filament of the solitary staminate flower 

 elongates to raise the anthers above the 

 stigmas of the pistillate flowers. The pollen is heavy enough to 

 slowly sink after escaping from the stamens and in still water 

 may pollinate the flower of its own plant, but in running water is 

 usually carried to a neighboring plant. 



The pollen grains of aquatic plants differ in one particular from 

 those of land forms in that they have only one coat. Perhaps this 



The Fig. 267. Zannichellia palustris. 

 Pollination occurs under water. 

 Anthers are raised above the 

 stigmas by the long filament. 

 X about 8. (After Gobel.) 



