NAIAD ACE/E. 321 



Order XCVII. NAIADA'CE^. (Pondwebd Family.) 



Immersed aquatic herbs, -with, jointed stems and sheathing 

 stipules. Leaves flat, immersed or floating. Flowers per- 

 fect or imperfect, inconspicuous, naked, or with a free scale- 

 like calyx. Ovaries 1-oelled, 1-seeded, solitary or 2-4, and 

 distinct. 



Synopsis of the Gciicrn. 

 * Flowers perfect^ in spikes or clusters. 



1. Potninoge'ton. Flowers with sepals, stamens, and sessile ovaries. 



Each 4. 



* * Flowers tnoncecioua or dioecious^ axillary^ naked^ monatidrozts. 



2. Nai'as. Flowers diceclous. Ovaries solitary and naked. Stigmas 2 



or 4, awl-shaped. Staminate flower (of 1 stamen) enclosed In a 

 membranous spathe. Stems floating. Leaves linear, opposite, 

 somewhat crowded into whorls, sharp-toothed, sessile and spread- 

 ing at the base. 



3. Zauiilclicll'ia. Flowers monceclons, usually both kinds in the same 



axil. Oearies 2-5, from a cup-shaped Involucre. Leaves linear- 

 thread-form, entire. 



4. Zoste'ra. Flowers monoecious, the two kinds naked and sessile, 



arranged alternately in 2 vertical rows on the inner side of a leaf- 

 like enclosed spadlx. Sterile flowers of single ses.sile l-celled 

 anthers ; the fertile of single ovate-oblong ovaries. Stigmas 2, 

 bristle-form, deciduous. 



* 1. POTAMOGE'TOJT, Tourn. PONDWEED. 



* Leaves of 2 sorts, tlie fioating ones with a dilated petioled blade, differ- 

 ■ ent informfrom the subvierged ones. 



1. P. na,ta.ns, L. Submerged leaves grass-like or capillary ; 

 upper stipules very long, acute. Spikes cylindrical, all out 

 of the water. Stem hardly branched. Floating leaves long- 

 petioied, elliptical, with a somewhat heart-shaped base, with 

 a blunt apex, 21-29-nerved. 



2. P. ClaytO'nii, Tuckerman. Stem compressed. Sub- 

 merged leaves linear, 2-5 inches long, 2-ranked, 5-nerved ; 

 stipules obtuse. Floating leaves short- petioled, chiefly oppo- 

 site, oblong, 11-17-nerved. Spikes all above water. 



3. P. Spiril'lUS, Tuckerman. Stems very slender. Float- 

 ing leaves when present oval to lanceolate, about as long as 



