444 NAIADACE^. (rONDWEED FAMILY.) 



Order 138. NAIADACE^E. (Pondweed Family.) 



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

 monoecious or dioecious 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, or coiled. — Leaves sheathing, or with 

 sheathin<T stipules. Flowers commonly enclosed in a spathe. 



Synopsis. 



• Flowers monoecious or dioecious. 



1. NAIAS. Stigmas 2-4. Flowers naked. Leaves opposite or whorled. 



2. ZOSTERA. Stigmas 2. Flowers enclosed in a spatbe. Leaves alternate. 



8. ZANNlCliELLlA. 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 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 erect. 

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

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

 Flowers solitary, minute. 



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

 narrow-linear, membranaceous, spreading, minutely dciuiculatc on the margins, 

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

 yellowish. (Caulinia flexilis, Willd.) — In ponds and still water. South Caro- 

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



Var. "* fusiformis. Stem (6' -12' long) almost capillary, very leafy 

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

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

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



2. ZOSTERA, L. Eel-grass. 



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

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

 Bhcath-like base of the leaves. Anthers oblong, 1-cclled, filled with fine filament* 

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

 dulous orthotropous ovule, an<l pointed with the subulate persistent style. Stig 

 mas 2, caf)illary. Utricle bursting irregularly. Seeds striate. Cotyledons in 

 flexed curved, received in a lonu'itudiiial cleft of the embryo — Marine herbs 

 with creeping stems, and nunowly liueur obtuse uiid elongated bheatliing leaves, 



