NAIADACEjE. 221 



ORDER XCVII. NAIADA'CE^E. (PONDWEED 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-celled, 1-seeded, solitary or 2-4, and 

 distinct. 



Synopsis of the Geiiera. 



* Flower s per feet, in spikes or clusters. 



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



ach 4. 



* * Flowers monoecious or dioecious, axillary, naked, monandrous. 



2. Xai'as. Flowers dioecious. 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. ZannicheH'la. Flowers monoecious, usually both kinds in the same 



axil. Ovaries 2-5, from a cup-shaped involucre. Leaves liuear- 

 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 spadix. Sterile flowers of single sessile l-cel!ed 

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

 bristle-form, deciduous. 



1. POTAMOGE'TON, Tourn. PONDWEED. 



* Leaves of 2 sorts, the floating ones with a dilated pet ioled blade, differ- 

 ent in form from the submerged ones. 



1. P. natans, 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- 

 petioled, 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 



