ZANNICHELLIACEAE 37 



long, attenuate : sheaths membranous, 6-8 mm. long, each with a minute ligule : peduncles 

 filiform, sometimes becoming 3 dm. long : drupelets ovoid, about 2 mm. long, oblique or 

 gibbous at the base, 4-7 together, in umbel-like clusters, each on a stipe 1-4 cm. long. 



In salt or brackish water on the Atlantic and Pacific coasts and in the interior of North America. 

 Summer. Widely distributed throughout the world. DITCH-GRASS. 



3. FOTAMOGETON L. 



Perennial submerged herbs, with more or less elongated often branching stems. Leaves 

 commonly numerous, often of 2 kinds : submerged of thin texture : floating smaller, of 

 thick texture. Spathe stipule-like, free, or united with the base of the petiole or blade, 

 early deciduous. Peduncles axillary. Spikes mostly erect. Flowers perfect. Perianth 

 wanting or obsolete. Stamens 4. Anthers sessile, the connectives bearing dilated appen- 

 dages. Pistils 4, distinct, sessile : ovary 1-celled. Stigmas sessile or terminating recurved 

 styles. Fruit of 4 drupelets with a hard or spongy pericarp. Seeds solitary, crustaceous, 

 campylotropous. Embryo hooked. PONDWEED. 



Stipules axillary, free from the leaf. 



Plants with both submerged and floating leaves. 

 Submerged leaves without blades. 



Floating leaves with ovate broadly elliptic or oval blades subcordate or rounded at the base : 



flowering spikes 3-6 cm. long, 5-6 mm. thick. 1. P. natans. 



Floating leaves with narrowly elliptic blades acute at both ends : flowering 



spikes 1-2 cm. long, 2.5-4 mm. thick. 2. P. Floridanus. 



Submerged leaves with normal blades. 



Blades of submerged leaves of 2 forms. 3. P. pulcher. 



Blades of submerged leaves all alike. 



Submerged leaves with linear or nearly linear blades. 



Leaf-blades with coarse cellular reticulation in the middle. 4. P. Nuttallii. 



Leaf-blades with inconspicuous cellular reticulation. 5. P. heterophyllus. 



Submerged leaves with lanceolate or oblanceolate blades. 



Leaf-blades distinctly petioled. 6. P. Lonchites. 



Leaf-blades sessile or nearly so. 7. P. Zizii. 



Plants with submerged leaves only. 

 Leaf-blades broad. 



Blades short-petioled or nearly sessile. 8. P. lucens. 



Blades clasping the stem. 9. P. perfoliatus. 



Leaf-blades linear or filiform. 



Blades 1-nerved. 10. P. Curtism. 



Blades 3-nerved. 



Drupelets crested : plants without propagating buds or glands. 11. P. foliosus. 



Drupelets crestless : plants with both propagating buds and glands. 12. P. pusillus. 

 Stipules adnate to the blades or the petioles of the submerged leaves. 



Plants with both submerged and floating leaves : drupelets crested. 13. P. diversifolius. 



Plants with submerged leaves only : drupelets crestless. 14. P. pectinatus. 



1. Potamogeton natans L. Stems 6-12 dm. long, simple or sparingly branched : 

 leaves various, of 2 kinds, those of submerged leaves reduced to bladeless petioles which 

 perish early ; those of floating leaves with leathery ovate broadly elliptic or oval blades 4-9 

 cm. long, usually abruptly pointed, many-nerved, rounded or subcordate at the base ; 

 petioles as long as the blades or much longer : peduncles as thick as the stem, 4-9 cm. 

 long : spikes cylindric, 3-6 cm. long, dense : drupelets narrowly obovoid, 4-4.5 mm. long, 

 scarcely keeled : nutlets more or less pitted on the sides, 2-grooved on the back. 



In still water or streams, nearly throughout North America. Also in Europe and Asia. Summer. 



2. Potamogeton Floridanus Small. Kesembling P. natans, but more slender. 

 Floating leaves few ; blades narrowly elliptic, 4-7 cm. long, acute at both ends, narrowly 

 revolute, with 7 main nerves ; petioles very slender, several times longer than the blades : 

 peduncles stouter than the stem, 4-6 cm. long : flowering spikes cylindric, 1-2 cm. long, 

 2.5-4 mm. thick. 



In the Blackwater River, Florida. Spring and summer. 



3. Potamogeton pulcher Tuckerm. Stems 3-6 dm. long, nearly simple, terete, 

 spotted : leaves various ; blades of 3 kinds, those of submerged leaves leathery, spatulate 

 ovate or oblong, and those above these flimsy, pellucid, lanceolate, 8-20 cm. long, attenu- 

 ate, short-petioled ; those of floating leaves leathery, ovate or orbicular-ovate, 4-10 cm. long, 

 obtuse or abruptly pointed, many-nerved, cordate at the base ; petioles as long as the blades 

 or longer : peduncles slightly thicker than the stem, 5-10 cm. long : spikes cylindric, dense, 

 2-2.5 cm. long : drupelets turgid, 4-4.5 mm. long, abruptly pointed, 3-keeled on the back. 



In ponds, Maine to Missouri and Georgia. Summer. 



4. Potamogeton Nuttallii Cham. & Schlecht. Stem slender, 3-20 dm. long, flattened, 

 simple or sparingly branched : leaves various ; blades of 2 kinds, those of submerged leaves 

 linear, 2-ranked, 5-16 cm. long, 5-nerved, reticulated between the midrib and contiguous 



