15. Potamogeton natans L. Broad-leaved pondweed. Fig. 45. 



Stems branching from a horizontal rhizome, otherwise usually simple; sub- 

 mersed leaves without blades, 1-3 dm. long. 0.8-2 mm. wide, rarely with a 

 poorly developed blade, the linear stipules 6-8 cm. long; floating leaves broadly 

 elliptic to oblong, often subcordate at base, broadly rounded at apex, 25- to 27- 

 nerved, the petiole longer than blade, the stipules 5-12 cm. long, linear-lanceolate, 

 membranous; spikes in the axils of floating leaves, 3-6 cm. long on stout pe- 

 duncles, P/i to 3 times as long as the spike; nutlets 3-5 mm. long, strongly keeled 

 on the back, the lateral angles scarcely evident, the beak erect. 



Marshy ponds and lakes, often brackish, in Okla. (Choctaw Co.), N. M. (San 

 Juan and Sandoval cos.) and Ariz. (Apache and Coconino cos.), May-Sept.; 

 Greenl. to Alas., s. to N. J., Pa., O., Ind., 111., la.. Neb., N.M., Ariz, and Calif. 



A sterile specimen in the U. S. National Herbarium might possibly be P. 

 alpinus Balbis. It was collected by R. O. Studhalter, etc. (S3874) at Glacial Lake 

 near Tres Ritos, Taos Co., New Mexico, at 9,500 ft. elevation. It is distinguished 

 from P. amplifolius, which it superficially resembles, by its usually smaller, sessile, 

 submersed leaves, more slender rhizome and usually reddish stems and peduncles. 

 Its floating leaves, when present, are also delicate and thin with no sharp distinc- 

 tion between blade and petiole. 



Fam. 16. Zannichelliaceae Dum. Horned Pondweed Family 



Submerged aquatic dioecious or monoecious herbs, with a slender creeping 

 rhizome; leaves alternate or opposite or crowded at the nodes, linear, sheathing 

 at the base, the sheaths mostly ligulate at the apex, the floral leaves sometimes 

 reduced to sheaths; flowers minute, bisexual or unisexual, axillary, solitary or in 

 cymes; perianth of 3 small free scales or absent; stamens 1 to 3, the anthers 1- or 

 2-celled and opening lengthwise; pollen globose or threadlike; gynoecium of 1 to 

 9 free carpels; style short or long, simple and with a capitate to peltate or spatu- 

 late stigma, sometimes 2- to 4-lobed; ovule solitary, pendulous; fruiting carpels 

 sessile or stipitate, indehiscent; seed pendulous, without endosperm. 



Widely distributed, mainly in salt or brackish water; 3 genera and 6 species. 



1. Pollen spheroid; carpels several, free; plants of fresh or brackish water; leaves 



filiform 1. Zannichellia 



1. Pollen threadlike; gynoecium 1- or 2-carpenate; plants of marine habitats (2) 



2(1). Leaves flat, tridentate at apex; styles simple; one anther attached higher 

 than the other 2. Halodule 



2. Leaves terete or semiterete, acute or pointed at apex; styles 2- to 4-lobed; 



anthers at an equal height 3. Cymodocea 



1. Zannichellia L. Horned Pondweed 



A genus of two species, the other in Africa. Placed by some authors in the 

 Najadaceae. 



1. Zannichellia palustris L. Common poolmat. Fig. 46. 



Submerged aquatic plant, monoecious, rooted on bottom and floating below 

 surface of water; rhizome creeping; stem slender, simple or much-branched; leaves 

 mostly opposite, linear-filiform, entire, to 1 dm. long, acute or almost pungent 

 at the apex, 1 -nerved; stipules scarious, free from the leaf bases, scarcely 2 cm. 

 long; flowers unisexual, sessile, usually both kinds from the same axil, enclosed in 

 a hyaline deciduous spathe, the perianth wanting; staminate flower consisting of 



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