FIELD BOOK OF PONDS AND STREAMS 



face. The short-stemmed male flowers develop near the 

 base of the plant, several hundreds of them in each cluster, 

 the separate flowers becoming detached as they mature. A 

 bubble of air in each one buoys it to the top and there it opens 

 and floats along the surface till it meets the female flower. 

 The male flowers congregate in great numbers about the 

 larger female ones to which their sticky pollen finally adheres. 

 After pollination the spiral stem of the female flower con- 

 tracts and pulls the maturing fruit down into the water. 



Occurrence. — Common in quiet waters. August. Eastern 

 states to South Dakota, and Texas; occasional on Atlantic 

 Coastal Plain. 



Animal associates. — Fresh water eel-grass is the favorite 

 food of the canvas-back duck and other water birds. It is 

 an excellent aquarium plant, growing easily and providing 

 abundant oxygen for the animals (Fig. 35). 



Duckweeds — Lemnacece 



All the duckweed family are very small and all float upon 

 the surface of the water; among them are the Wolffias (Fig. 

 70), the smallest of the flowering plants. They have no true 

 leaves nor stems but the green plant body or thallus is usually 

 called a leaf and it looks very much like one. Minute flowers 

 grow directly out of it, borne from its side or upper surface. 

 The male flower is only a single stamen, and the female flower 

 a single pistil. Both are rare, for duckweeds usually repro- 

 duce by "budding off", or division of the thallus into two 

 parts, each of which produces roots and becomes a new 



plant. 



Greater duckweed, Spirodela polyrhiza.— In many regions 

 this species (Fig. 67) is the common duckweed, forming the 

 green water blankets made of thousands of little plants float- 

 ing on the surface, each with 4 to 16 rootlets hanging from it. 

 The thallus is circular, usually about a quarter of an inch 

 wide, green above and pinkish purple beneath; its 5 to 15 



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