Fig. 43: Potamogeton illinoensis: a, habit, showing profusion of crowded leaves, 

 conspicuous stipules and long stout peduncles, X %; b, part of flowering spike, X 4; c, 

 achene, showing strong dorsal keel, smooth face and short beak, X 6; d, achene 

 (longitudinal section), X 6. (From Mason, Fig. 20). 



