362 NAIADACE^E. (PONDWEED FAMILY.) 



* Immersed aquatics with flat leaves : ovaries 4, distinct. NAIADEJE. 



1. Zanichellia. Flowers monoecious, axillary. Stamen 1, with slender filament. Fertile 



flowers solitary, with a cup-shaped membranous spathe or perianth. Ovaries nearly 

 sessile, becoming more or less stipitate : stigmas peltate. Leaves opposite. 



2. Potamogeton. Flowers perfect, with herbaceous 4-sepaled perianth, in a peduncled 



spike. Anthers 4, sessile. Ovaries sessile : stigma sessile, unilateral. Leaves mostly 

 alternate. 



* * Marsh plants with terete bladeless leaves : flowers perfect, spicate or racemose, with 

 herbaceous 6-lobed perianth : carpels more or less united, separating at maturity. 



JUNCAGINE^E. 



3. Triglocliin. Ovaries 3 to 6, united until maturity. Leaves radical. Flowers bract- 



less, in a spike-like raceme terminating a jointless scape. 



4. Scheuchzeria. Ovaries 3, nearly distinct, at length divergent. Flowers bracteate in 



a, loose raceme upon a leafy stem. 



1. ZANICHELLIA, Micheli. HORNED PONDWEED. 



Flowers sessile or nearly so. Male flowers of a single naked stamen. Fertile 

 flowers usually in the same axils. Fruit an obliquely oblong beaked nutlet. 

 Very slender and branching, with very narrow and filiform leaves, not 

 sheathing and with small stipules. 



1. Z. palustris, L. Stems 2 inches to 2 feet long or more, leafy : leaves 

 to 3 inches long : fruit somewhat incurved, often more or less toothed on the 

 back. From New Mexico and S. Colorado northward, and in both the Pacific 

 and Atlantic States. In fresh-water ponds and slow streams. 



2. POTAMOGETOW, Tourn. PONDWEED. 



The four stamens opposite the perianth segments. Fruit somewhat com- 

 pressed, ovate, drupe-like, with a crustaceous nutlet within. Slender, jointed 

 and branching, in fresh or brackish water, with linear or dilated leaves, and 

 scarious stipules : spikes enclosed in the bud, at length long-exserted. 1 

 * Floating leaves more or less coriaceous, with a dilated petioled blade, different 

 in form from the thinner submerged ones; stipules free: spikes cylindrical, 

 mostly dense, not interrupted. 

 H- Submerged leaves reduced to narrow! >/ grass-like orjiliform sessile phyllodia. 



1. P. natans, L. Stem rather stout, simple or sparingly branched: 

 floating leaves thick, ovate-elliptic to lanceolate, acutish, slightly cordate at 

 base, 21 to 29-nerved, mostly shorter than the petiole ; stipules long and con- 

 spicuous; upper submerged leaves with a small lanceolate blade, the lower 

 (formed early or late in the season) reduced to phyllodia: peduncle stout, 

 bearing an emersed spike : fruit turgid, obliquely obovate, acute : nutlet with 

 a small deep pit on each side. Across the continent, in ponds and ditches. 

 In deeper or flowing water, the plant becomes more slender and often sub- 

 merged. 



<- *- Submerged leaves lanceolate, rarely oi:al or linear. 



2. P. rufescens, Schrad. Floating leaves (often wanting) rather thin, 

 11 to 17 '-nerved, narrowly oblong-elliptic or oblanceolate, acutish, attenuate into 



1 Mature fruit is necessary for positive determination. 



