444 NAIADACE^. (POND WEED FAMILY.) 



Order 138. NAIADACEiE. (Pondweed Family.) 



Aquatic herbs, with slender jointed leafy immersed stems, and perfect 

 monoecious or dioecious flowers, destitute of floral envelopes, or with 

 scale-like sepals. 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, or coiled. — Leaves sheathing, or with 

 sheathing stipules. Flowers commonly enclosed in a spathe. • 



Synopsis. 



* Flowf rs monoecious or dioecious. 



1. NAIAS. Stigmas 2-4. 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 monoecious or dioecious, axillary, sessile, destitute of calyx and co- 

 rolla. Sterile Hower 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 erect. 

 Achenium minute. Embryo straight. Radicle inferior. — Stems filiform, fork- 

 ing. Leaves opposite or whorled, linear, dilated into a short sheath at the base 

 Flowers solitary, minute. 



1. N. flexilis, Eostk. Stem immersed (1°- 2° long) ; leaves 3 in a whorl, 

 nan-ow-linear, membranaceous, spreading, minutely denticulate on the margins, 

 the lower ones often remote ; stigmas 3 - 4 ; achenium elliptical, acute, smooth, 

 yellowish. (Caulinia flexilis, Willd.)—Jn ponds and Stillwater, South Caro- 

 lina, and northward. July and Aug. — Leaves about 1' long. 



Van? fusiformis. Stem (6' -12' long) almost capillary, very leafy 

 throughout ; leaves opposite, approximate, spreading or recurved (2" -4" long) ; 

 achenium narrowly spindle-shaped, finely reticulated, brownish. — Brackish 

 water along the coast. West Florida. July and Aug. 



2. ZOSTERA, L. Eel-gkass. 



Flowers monoecious, naked ; the sterile and fertile ones alternately arranged 

 on the anterior edge of a flattened membranaceous spadix, and enclosed in the 

 sheath-like base of the leaves. Anthers oblong, 1 -celled, filled with fine filaments 

 instead of pollen-grains. Ovary fixed near the apex, containing a single pen- 

 dulous orthotropous ovule, and pointed with the subulate persistent style. Stig- 

 mas 2, capillary. Utricle bursting irregularly. Seeds striate. Cotyledons in- 

 flexed-eurved, received in a longitudinal cleft of the embryo — Marine herbs, 

 with creeping stems, and narrowly linear obtuse and elongated sheathing leaves. 



