444 NAIADACEJE. (PONDWEED FAMILY.) 
OrDER 138. NAIADACEZJE. (Ponpweep FAMILY.) 
Aquatie herbs, with slender jointed leafy immersed stems, and perfect 
monecious or diecious flowers, destitute of floral envelopes, or with a 
scale-like calyx. Stamens 1—4: anthers 1—4-celled. Ovary 1-celled, 
forming a 1-seeded achenium in fruit. Stigmas 1-4. Seed without al- 
bumen. Embryo straight, curved, ar coiled. — Leaves sheathing, or with 
sheathing stipules. Flowers commonly enclosed in a spathe. 
Synopsis. 
* Flowers monecious or dicecious. 
l. NAIAS. Stigmas2-4. Flowers naked. Leaves opposite or whorled. 
2. ZOSTERA. Stigmas 2. Flowers enclosed in a spathe. Leaves alternate. 
3. ZANNICHELLIA. Stigma single, peltate. Ovaries 4, from a cup-like involucre. 
* * Flowers perfect. . 
4. RUPPIA. ° Fruit long-peduncled, umbellate. Calyx and corolla none. 
5. POTAMOGETON. Fruit sessile, spiked. Calyx 4-leaved. 
1, NAIAS, L. 
Flowers moneecious or dicecious, axillary, sessile, destitute of calyx and co- 
rolla. Sterile flower monandrous, enclosed in a spathe. Anther 4-celled, open- 
ing at the apex, the filament lengthening. Fertile flower naked; the sessile 
ovary pointed with the slender style. Stigmas 2-4, subulate. Ovule erc. —— 
Achenium minute. Embryo straight. Radicle inferior. Stems filiform, fork- 
ing. Leaves opposite or whorled, linear, dilated into a short sheath at the pes 
Flowers solitary, minute. 
1." N. flexilis, Rostk. Stem immersed (1? - 2° long) ; leaves 3 ina who. 
narrow-linear, membranaceous, spreading, minutely denticulate on the margins 
the lower ones often remote ; stigmas 3-4 ; achenium elliptical, acute, smo0" 
yellowish. (Caulinia flexilis, Willd.) — In ponds and still water, South Caro 
lina, and northward. July and Aug. — Leaves about 1’ long. eth 
Var. ? fusiformis. Stem (6’-12’ long) almost capillary, VY eed 
throughout ; leaves opposite, approximate, spreading or recurved ar 
achenium narrowly spindle-shaped, finely reticulated, brownish. — aie 
water along the coast. West Florida. July and Aug. 
2. ZOSTERA, L. Exr-crass. T 
. Flowers monecious, naked; the sterile and fertile ones alternately arange 
ms, and narrowly linear obtuse and elongated sheathing 
