PONDWEED FAMILY 69 



Pistils many, borne on the side of a linear spadix; maritime. 



Flowers monoecious; nutlet ovoid; leaves 2 to 4 lines broad 4. ZOSTLRA. 



Flowers dioecious ; nutlet sagittate-cordate ; leaves % to 2 lines broad 



5. PHYLLOSPADIX. 



Leaves with spiny-toothed margins ; pistil solitary and naked 6. NAIAS. 



1. POTAMOGETON L. PONDWEED. 



Perennial herbs, commonly growing in the still waters of creeks and in fresh 

 or brackish ponds, the stems arising from rootstocks. Leaves alternate, or the 

 uppermost opposite, frequently of two kinds, the floating ones broad, the sub- 

 merged narrower and often thread-like or linear; stipules present, often 

 sheathing the stem. Flowers in spikes or heads on axillary peduncles and 

 enclosed in the bud by stipular sheaths. Sepals 4, with short claws. Stamens 

 4, inserted on the base of sepals. Ovaries 4. About 60 species, in all parts of 

 the earth. (Greek potamos, a river, and geiton, a neighbor, on account of the 



aquatic habit.) 



A. Stipules axillary and free from the leaf. 



Plants with both submerged and floating leaves; petioles of floating leaves present, often long, 



short or none in no. 4. 

 Submerged leaves linear or thread-like, consisting of petioles only. 



Floating leaves elliptical, subcordate at base 1. P. natans. 



Floating leaves narrowly oblong, attenuate into the petiole 2. P. epihydrus. 



Submerged leaves linear or lanceolate, bearing true blades. 



Flowers capitate; peduncles 1 to 3 lines long; floating leaves less than 1 inch long 



3. P. dimorphus. 

 Flowers spicate; peduncles 2 inches long or more; floating leaves 2 to 4 inches long. 



Plants reddish; nutlet with a distinct pit on each side 4. P. alpinus. 



Plants green; nutlet not pitted. 



Nutlet distinctly 3-keeled; low altitudes 5. P. americanus. 



Nutlet indistinctly 3-keeled; high montane 6. P. heterophyllus. 



Submerged leaves, or some of them, broader and falcate 7. P. amplifolius. 



Plants with the leaves all submerged; petioles short or none. 



Leaves with broad blades, ovate, orbicular or lanceolate, never linear. 



Stipules greenish ; leaves with a short petiole or subsessile 8. P. lucens. 



Stipules white, with numerous fibrous nerves. 



Leaves clasping, hooded at apex; peduncles often 8 inches long or more 



9. P. praelongus. 



Leaves cordate-clasping, not hooded, the lobes at base often touching around the stem. 



10. P. perfoliatus. 

 Leaves linear, thread-like, or setaceous. 



Without propagating buds or glands 11. P. foliosus. 



With both propagating buds and glands. 



Leaves capillary ; stem slender, not flattened 12. P. pusillus. 



Leaves linear, 1 to 2 lines wide; stem much flattened .13. P. compressus. 



B. Stipules adnate to the leaf or petiole. 

 Plants with submerged leaves only. 



Leaves capillary 14. P. pectlnatus. 



Leaves flat, % to 1% lines broad. 



Leaves in terminal clusters 15. P. latifolius. 



Leaves 2-ranked 16. P. robbinsii. 



1. P. natans L. BROAD PONDWEED. Stem thick, little if at all branched; 

 floating leaves elliptical, subcordate at base, 1^ to 3 inches long, 1 to 2 inches 

 broad, on petioles longer than the blade ; stipules linear-lanceolate, membran- 

 aceous, 2 to 4 inches long; submerged leaves consisting of petioles without 

 blades, 2 to 9 inches long or more and 1 line wide, usually perishing early, their 

 tips sometimes reaching the surface of the water and forming miniature blades ; 

 spikes dense, 1 or 2 inches long, on longer peduncles ; nutlet evidently keeled 

 along the back, 2 lines long. 



