NAIADACE^E. (PONDWEED FAMILY.) 445 



I. Z. marina, L. Stem slender, terete, jointed; leaves thin and tender, 

 faintly 3-5-nerved; flowers in two rows on the linear spadix. Deep salt- 

 water coves. West Florida, and northward. Aug. and Sept. 



3. ZANNTCHELLIA, L. 



Flowers monoecious, axillary. Sterile flower consisting of a solitary naked 

 filament bearing a 2 - 4-celled anther. Fertile flower from the same axil, com- 

 posed of 2-6 sessile 1 -celled ovaries, surrounded by a cup-shaped involucre, and 

 pointed with the slender style. Stigma obliquely peltate. Ovule suspended, 

 orthotropous. Achenium oblong, stalked. Embryo slender, coiled. Sub- 

 merged aquatic plants, with filiform branching stems, and very narrow and 

 entire alternate leaves, with sheathing stipules. 



1. Z. palustris, L. Stems tufted, filiform, alternately branched; leaves 

 narrowly linear, entire, acute, 1 -nerved ; anther 2-celled, on a long and slender 

 filament ; achenia 3-6 in a cluster, commonly raised on a short common pe- 

 duncle, each stalked, linear-oblong, somewhat compressed and curved, smooth 

 and wingless, about one third longer than the persistent style. Fresh or brack- 

 ish water, West Florida, and northward. May -Aug. Stems l-3 long. 

 Leaves 1' - 2' long. 



4. RTTPPIA, L. DITCH-GRASS. 



Flowers perfect, naked, two or more on a slender spadix, enclosed in the 

 spathe-like sheaths of the leaves, but soon long-exserted. Stamens 2, closely 

 sessile : anther-cells large, distinct. Ovaries 4, sessile, containing a single sus- 

 pended campylotropous ovule. Stigma peltate. Achenium stalked, obliquely 

 ovate. Embryo pointed by the short plumule. Salt-water herbs, with filiform 

 branching stems, and alternate linear or bristle-like sheathing leaves. 



1 . R. maritima, L. In shallow water, along the coast, Florida, and 

 northward. May -Aug. Stems immersed, l-3 long, mostly creeping at 

 the base. Leaves filiform, l'-3' long, with dilated membranaceous sheaths. 

 Fruiting peduncles l'-4' long. Achenium pointed. 



5. POTAMOGrETON, Tourn. PONDWEED. 



Flowers perfect, spiked. Sepals 4, roundish, valvate in the bud. Stamens 

 4, opposite the sepals : filaments short : anthers 2-celled. Ovaries 4, sessile. 

 Ovules ascending, campylotropous. Style short or none : stigma peltate. 

 Achenia 1-4, compressed. Embryo curved or coiled. Aquatic herbs, with 

 immersed slender and jointed stems. Leaves stipulate, alternate and opposite, 

 either all immersed and commonly membranaceous, or the upper ones floating 

 and more rigid. Spikes peduncled, axillary and terminal. 

 * Leaves all immersed and alike. 



<- Leaves filiform. 



1. P. pectinatus, L. Stem slender, flexnous ; the branches diffusely 

 forking, distichous ; leaves long, thickish, slightly channelled, approximate on 

 38 



