NAIADACEAE (PONDWEED FAMILY) 33 



12. NAIADACEAE Lindl. PONDWEED FAMILY 



Marsh or mostly immersed aquatic plants with jointed, leafy stems, and 

 the leaves sheathing at base or stipulate. Flowers perfect, monoecious or 

 dioecious, often, spathaceous. Perianth of 4 segments, or membranaceous, or 

 wanting. Stamens 1-4. Ovaries 1-9, distinct, 1-celled, 1-ovuled. Naiadaceae 

 in part. 



Flowers perfect; leaves alternate; anthers 2 or 4, sessile. 



Perianth-segments 4, distinct; ovaries sessile . . . . .1. Potamogeton. 



Perianth wanting; ovaries at length stipitate . . . . .2. Ruppia. 

 Flowers monoecious or dioecious; leaves opposite or fascicled; stamen 

 solitary. 



Leaves entire; flowers clustered in the axils 3. Zannichellia. 



Leaves spiny- toothed; flowers solitary in the axils . . . .4. Naias. 



1. POTAMOGETON L. PONDWEED 



Herbs of fresh or saline ponds and streams, with jointed mostly rooting 

 stems, and 2-ranked leaves, which are usually alternate or imperfectly op- 

 posite: the submersed ones pellucid, the floating ones often dilated and of 

 firmer texture. Stipules membranous, more or less united and sheathing. 

 Spikes sheathed by the stipules in the bud, mostly later raised on a peduncle 

 to the surface of the water. Flowers perfect. Sepals 4, rounded, valvate in 

 the bud. Stamens 4, opposite the sepals; anthers nearly sessile, 2-celled. 

 Ovaries 4 (rarely only one), with an ascending campylotropous ovule; stigma 

 sessile or on a short style. Fruit drupe-like when fresh, more or less com- 

 pressed; endocarp (nutlet) crustaceous. 



Some of the leaves floating. 



Floating leaves elliptic to oblanceolate, petioled. 



Submerged leaves bladeless . . . . . . 1. P. natans. 



Submerged leaves with a proper blade. 



Of two forms elliptic and lanceolate 2. P. amplifolius. 



Of one form all alike. 

 Linear. 



Filiform, with adnate stipules . . . . 3. P. diversifolius. 



Broadest at base; with free stipules . . . 4. P. heterophyllus. 



Not linear. 



Petiole of floating leaves long (3-10 cm.) . . 5. P. lonchites. 



Petiole of floating leaves short (3-15 mm.). 



Blade elliptic 6. P. Zizii. 



Blade oblanceolate . . . . . . 7. P. alpinus. 



None of the leaves floating. 



Leaves with lanceolate or broader blade. 



Amplexicaul 8. P. perfoliatus. 



Sessile or nearly so ........ 9. P. lucens. 



Leaves linear. 



Stipules free; style present. 



Leaves glandular at base ....... 10. P. pusillus. 



Leaves not glandular at base 11. P. foliosus. 



Stipules adnate. 



Style present, deciduous; fruit plump . . . . .12. P. pectinatus. 

 Style wanting; stigma sessile, persistent . . . . 13. P. filiformis. 



1. Potamogeton natans L. Sp. PI. 126. 1753. Stem rather stout, simple 

 or sparingly branched: floating leaves thick, ovate-elliptic to lanceolate, 

 acutish, slightly cordate at base, 21-29-nerved, mostly shorter than the peti- 

 ole; stipules long and conspicuous; upper submerged leaves rarely with a 

 small lanceolate blade, the lower (formed early in the season) reduced to 

 phyllodia: peduncle stout, bearing an emersed spike: fruit turgid, obliquely 

 obovate, acute: nutlet with a small deep pit on each side. Across the con- 

 tinent, in ponds and ditches. In deeper or flowing water the plant becomes 

 more slender and often wholly submerged. 



2. Potamogeton amplifolius Tuckerm. Am. Journ. Sci. II. 6: 225. 1848. 

 Stems often stout, simple: floating leaves (sometimes wanting) 30-50-nerved, 

 elliptic to oblong-lanceolate, acute, mostly rounded or slightly cordate at 



ROCKY MT. EOT. 3 



