12 Potai7togetonaceae, Naiadaceae 



Perianth mostly absent. Stamens 4-1. Carpels 4-1, 

 each with one pendulous ovule. Fruit often drupaceous. 



a. Flowers in spikes. 



Tribe i. Zosterea£. Flowers diclinous in a flat, 

 dorsiventral spike enclosed in the sheath of the upper- 

 most foliage leaf. Pollen filiform. 



Only British genus : Zostera. 



Tribe 2, Potamoget07ieae. Axis of spike X.^xqX.q, flowers 

 monoclinous. 



Spike usually many-flowered. Tepals 4 : stamens 4 : 

 achenes sessile. Potamogeton. 



Spike i-2-flowered. Tepals o : stamens 2 : achenes 

 stipitate. Ruppia. 



b. Flowers solitary or cymose, diclinous. 



Tribe 3. Zannichellieae, Perianth absent or very 

 simple. Carpels 4-3. 



Only British genus : Zannichellia. 



Fam. 2. Naiadaceae. Slender, submerged fresh 

 or brackish water herbs. Stem with central cylinder of 

 elongated cells enclosing a canal Leaves opposite, 

 linear, toothed, almost imbricating. Flowers diclinous. 

 ^ of two cup-like envelopes and a terminal anther. 

 % of one cup-like envelope (which may be absent) and 

 one caipel containing one basal anatropous ovule. 



Only genus : Naias. 



*Fam. 3. Aponogetonaceae. Herbs with tu- 

 berous sympodial stems. Leaves submerged or lamina 

 floating. Spike aerial enclosed in deciduous spathe. 

 Flowers monoclinous. Perianth 3-1 petaloid. Stamens 

 6 or more. Carpels 3-6. Styles 3-6. Fruit leathery 

 with 2 or 00 seeds. 



Only genus : *Aponogeton. 



