KEY TO FAMILIES xxxi 



(m) Some or all of the leaves sub- 

 merged Naiadaceae 301 



(n ) Leaves usually aerial, occasion- 

 ally floating 

 r. Flowers in terminal cylindric 



spikes Typhaceae 300 



s. Flowers in lateral heads Sparganiaceae 301 



n. Leaves netted-veined Callitrichaceae 47 



