HIGHER PLANTS 



simple veins and a cluster of 5 to 10 rootlets distinguish this 

 species. 



^ W 



Fig. 67. — Greater duckweed, Spirodela polyrhiza. 



Occurrence. — Floating on the surface of pools, in marshes 

 and ponds, often very abundant in park and garden ponds. 

 June-early September. Common through most of the 

 United States, except toward the north, less frequent near 

 the coast. 



Animal associates. — As their name implies, these plants are 

 a favorite duck food. The}'" grow very rapidly, crowding 

 together in blankets which often so complet-ely shut out the 

 light from the water beneath that the submerged plants die 

 and the herbivorous insects and crustaceans are forced to 

 migrate for food (PL II, 2). 



Ivy-leaved duckweed, Lemna triscula. — The ivy-leaved 

 duckweeds (Fig. 68) form zigzag chains and lattices floating 

 just below the surface of the water. The thallus is oval or 

 lanceolate; their habit of remaining connected makes the 

 fanciful lattices. 



Occurrence. — In ponds and ditches, especially in cat-tail 



83 



