70 NAJADACEAE (PONDWEED FAMILY) 



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



1. Potamogeton. Spike peduncled. Sepals 4, herbaceous. Anthers 4. Ovaries 4, sessile. 



2. Ruppia. Flowers on an inclosed spadix, at length long-exserted, without perianth. Anther- 



cells 4, distinct. Ovaries 4, becoming stipitate. 



* * Flowers monoecious or dioecious, axillary, naked, monandrous ; leaves opposite (alternate 



in n. 4). 



3. Zannichellia. Monoecious. Pistils (2-5) from a cup-shaped involucre or sheath. 



4. Zostera. Pistils and stamens alternate in 2 vertical rows on the inner side of a leaf-like in- 



closed spadix. Stigmas 2, linear. Stem creeping. 



5. Najas. Dioecious. Pistil solitary, naked. Stamen inclosed in a membranous spathe. 



Stems floating, with opposite or ternate leaves. 



1. POTAMOGETON [Tourn.] L. POND WEED. 



Sepals 4, rounded, valvate in the bud. Stamens 4, opposite the sepals ; 

 anthers 2-celled. Ovaries 4 (rarely only one), with an ascending cainpylotro- 

 pous ovule ; stigma sessile or on a short style. Fruit drupe-like when fresh, 

 more or less compressed ; endocarp (seed) crustaceous. Embryo hooked, 

 annular, or cochleate, the radicular end pointing downward. Herbs of ponds 

 and streams, with jointed mostly rooting stems, and 2-ranked leaves, which are 

 usually alternate or imperfectly opposite ; the submersed ones pellucid, the 

 floating ones often dilated and of a firmer texture. Stipules membranous, more or 

 less united and sheathing. Spikes sheathed by the stipules in the bud, mostly 

 raised on a peduncle to the surface of the water. (An ancient name, composed 

 of 7roTa/x,6s, a river, and yeirwv, a neighbor, from the place of growth.) By 

 fruit, the full-grown fresh or macerated fruit is intended ; by seed, that with 

 the fleshy outer portion or epicarp removed. All measurements are from dried 

 specimens. The month mentioned indicates the time of ripening of the fruit. 



a. Leaves of two sorts ; floating ones more or less coriaceous, with a 

 dilated petioled blade, different in form from the thinner sub- 

 mersed ones &. 



6. Submersed leaves filiform or very narrowly linear, at most 2 mm. 



wide c. 

 c. Spikes all alike, cylindrical d . 



d. Blades of floating leaves 2.5 cm. or more long, mostly shorter 

 than the elongate petioles ; spikes 1.5 cm. or more long. 



Seed with a depression on each side 1. P. natcvns, 



Seed with plane sides, not at all impressed . . . 2. P. Oakesianus. 



d. Blades of floating leaves less than 1.5 cm. long, equaling or longer 



than the petioles ; spikes less than 1 cm. long. 

 Fruit compressed, distinctly keeled, tipped by the curved 



style 27. P. Vaseyi. 



Fruit plump, slightly grooved on the sides, but not keeled ; 



stigma nearly sessile 26. P. lateraUs. 



c. Spikes of two kinds ; one emersed, cylindrical, and many-flowered, 



the other submersed, globular, and few-flowered. 

 Peduncles of the submersed spikes equaling or exceeding the 



spikes 32. P. hybridus. 



Peduncles shorter than the submersed spikes . . . . 33. P. dimorphus. 

 I). Submersed leaves lanceolate to ovate, if linear more than 2 mm. wide e. 

 e. Submersed leaves linear and ribbon-like, with a broad coarsely 



cellular-reticulate space each side of the midrib . . . 4. P. epihydrus. 

 e. Submersed leaves broader /. 

 /. Principal floating leaves heart-shaped at base. 



Fruit 3-4 mm. long, compressed, and distinctly 3-keeled . . 7. P. pulcher. 

 Fruit 1.5-2 mm. long, plump, and obscurely 3-keeled . 3. P. polygonifolius 



f. Floating leaves rounded or tapering at base, not heart-shaped g. 



g. Floating leaves 30-50-nerved 8. P. amplifolius. 



g. Floating leaves with fewer nerves h. 

 h. Mature fruit 2.5 mrn. or more long i. 



i. Mature spikes 4-5.5 crn. long (if rarely shorter, with floating 

 leaves 18-24- nerved). 



Submersed leaves mucronate 11. P. angustifolius 



Submersed leaves merely acuminate. 

 Submersed leaves broadly lanceolate or oblong-ellipti- 

 cal ; fruit tipped by the prominent style . 9. P. illinoensis. 

 Submersed leaves narrowly lanceolate ; fruit tipped by 



the nearly sessile stigma 6. P. americanus. 



