146 Naiadaceae. 



IV. —SA GITTARIA. 



SAGITTA'RIASAGITTIFO'LIA(57). Arrowhead. 7—9. Flowers 

 (1473) monoecious, stamens and carpels numerous, leaves hastate 2 — 8 in. 

 erect, flowers \ in. diameter white about three in a whorl. Ditches. 



ORDER 82.— Naiadaceae. 



Aquatic plants rooting in the ground with parallel-veined leaves. Flowers 

 inconspicuous, perfect or monoecious. Perianth usually present, of 2 or 4 seg- 

 ments. Stamens I — 4 hypogynous. Ovary superior, of i — 4 carpels. Style 

 I. Stigma simple. Fruit i -celled i -seeded indehiscent. 



Stamens 4, Potamogeton I. 



Stamen i, . Zannichellia II. 



I. —POTAMOGE TON. 



Leaves submerged and transparent or floating and opaque alternate or 

 opposite, flowers perfect in axillary or terminal spikes, perianth-segments 4, 

 stamens 4, anthers nearly sessile. 



Leaves all submerged, 550 

 Upper leaves floating, 551 



550 Leaves opposite, ....... P. densus. 



550 Leaves alternate, 552 

 551 Submerged leaves sessile, 553 



551 Submerged leaves petiolate, ...... P. natans. 



552 Leaves linear, 554 



552 Leaves ovate or oblong, 557 

 553 Peduncles enlarged upwards, 555 



553 Peduncles not enlarged upwards, .... P. rufescens. 



554 Stipules adnate, ...... P. pectinatus. 



554 Stipules free, 556 



555 Stem stout, P. lucens. 



555 Stem slender, P. heterophyllus. 



556 Leaves under 2 in P. pusillus. 



556 Leaves over 2 in., . . . . .P. compressus. 



557 Stem flattened, P. crispus. 



557 Stem round, 558 



558 Leaves linear- oblong, peduncles long, . . P. pra;longus. 



558 Leaves ovato-cordate, peduncles short, . . P. perfoliatus, 



