444 NAIAD ACE.E. (POND WEED FAMILY.) 



ORDER 138. NAIADACE7E. (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-cclled, 

 forming a 1-seeded achenium in fruit. Stigmas 1-4. Seed without al- 

 bumen. Embryo straight, curved, qr coiled. Leaves sheathing, or with 

 sheathing 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 spathe. Leaves alternate. 



3. ZANNICIIELLIA. 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 whorled, 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 denticulate 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, finely reticulated, brownish. Brackish 

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



2. ZOSTERA, L. EEL-GBASS. 



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-curved, received in a longitudinal cleft of the embryo, Marine herbs, 

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



