70 NAJADACEAE (Pf )>.'DWE1<:d 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-ex serted, 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-hke in- 



closed spadix. Stigmas 2, linear. Stem creeping. 



5. Najas. Dioecious. Pistil solitary, naked. Stamen Inclosed in a membranonB spatbe. 



Stems floating, with opposite or ternate leaves. 



1. POTAMOG&TON [Tourn.] L. Pondwbbd. 



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

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

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

 more or less compressed ; endocarp (seed) crustac^ous. Embryo hooked, 

 annular, or cochleate, the radicular end pointing dov^^nward. • — 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 iroTafxSs, a rivei*, and yelroiifj a neighbor, from the place of growth.) — By 

 fruity 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. 



U-. 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 e. 

 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. nata/ns. 



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

 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. Vaaeyi, 



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



stigma nearly sessile 26. P. lateralis. 



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

 the other submersed, globular, and few-flowered. 

 Peduncles of the submersed spikes equahng or exceeding the 



spikes . . 82. P. hybridus. 



Peduncles shorter than the submersed spikes . . . . 38. P, dimorpMis. 

 h. Submersed leaves lanceolate to ovate, if linear more than 2 ram. 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-sha^ied at base. 



Fruit 3^ mm. long, compressed, and distinctly 3-fceeled . . 7. P. pulcher. 

 Fruit 1.5-2 mm. long, plump, and obscurely S-keeled . . 8. P. polygvnifoliu-a 

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



g. Floating i.eavesSO-Sn-nerved . 8. P. amplifoHuft. 



g. Floating leaves with fewer nerves h. 

 h. Mature fruit 2.5 mm. or more long /. 



*. Mature spikes 4-5 5 cm. long (if rarely shorter, with floating 

 leaves lS-24-nerved). 

 Submersed leaves mucronate .... . 11. P. angustifoUus. 



Submersed leaves merely acuminate. 

 Submersed leaves broadly lanceolate or oblong-ellipti- 

 cal ; fruit tipped by the prominent style , . 9. P. illinoenMft. 

 Submersed leaves narrowly lanceolate ; iVuittippt'd by 



the nearly sessile stigma . „ 6. P. americanus. 



