362 NAIAD ACE^. (POND WEED FAMILY.) 



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



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



flowers solil:.ry, with a cup-shaped menil)ranous 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. Autliers 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. — 

 Juncaginj;.e. 



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



less, in a spike-like raceme terminating a jointless scai>e. 



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



b. 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. POTAMOGETON, 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 h'near or dilated leaves, and 

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

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

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

 mosthj dense, not interrupted. 

 '*- Submerged leaves reduced to narroivhi grass-like or filiform sessile phijllodia. 



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. 



■t- +- Submerged leat^es lanceolate, rarely oval or linear. 



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

 11 fo \1 -nerved, narrowly oblong-elliptic or oblanceolate, acutish, attenuate into 



> Mature fruit is necessary for positive determination. 



