NAIADACEAE. 41 



lary or spadiceous flowers. Perianth of 4 segments, or a hyaline envelope, 

 or wanting. Stamens 1-4 or occasionally more, distinct and hypogynous 

 in the perfect flowers, solitary or connate in the sterile. Anthers ex- 

 trorse, i-2-celled. Ovaries 1-9, mostly distinct, i-celled, mostly i-ovuled. 

 Carpels rarely dehiscent. Seeds straight or curved. Endosperm none. 

 About 10 genera and 100 species of wide geographical distribution, most 

 abundant in temperate regions. The months noted in the descriptions 

 indicate the fruiting period. 



Flowers perfect. 



Perianth of 4 distinct segments. i. Potamogeton. 



Perianth none ; flowers naked. 2. Ruppia. 



Flowers monoecious or dioecious. 

 Leaves entire. 



Leaves i-nerved, 2-8 cm. long, 0.5 mm. or less wiJe. 3. Zannichellia. 



Leaves many-nerved, 0.3-1.6 m. long, 2-8 mm. wide. 4. Zostera. 



Leaves spiny-too'thed on the margins. 5. Naias. 



i. POTAMOGETON L. 



Leaves alternate or the uppermost opposite, often of 2 kinds, submerged and float- 

 ing, the submerged mostly linear, the floating coriaceous, lanceolate, ovate or oval. 

 Spathes stipular, often ligulate, free or connate with the base of the leaf or petiole, 

 enclosing the young buds and usually soon perishing after expanding. Peduncles 

 axillary, usually emersed. Flowers small, spicate, green or red. Perianth-segments 

 4, short-clawed, concave, valvate. Stamens 4, inserted on the claws of the perianth- 

 segments. Anthers sessile. Ovaries 4, sessile, distinct, I -celled, I-ovuled, attenu- 

 ated into a short style, or with a sessile stigma. Fruit of 4 drupelets, the pericarp 

 usually thin and hard or spongy. Seeds crustaceous, campylotropous, with an unci- 

 nate embryo thickened at the radicular end. [Greek, in allusion to the aquatic 

 habitat.] About 65 well-defined species, natives of temperate regions. Besides the 

 following, 3 others occur in the southern parts of N. Am. 



Stipules axillary and free from the leaf. 

 With floating and submerged leaves. 

 Submerged leaves bladeless. 



Nutlets more or less pitted. I. P. nafans. 



Nutlets not pitted. 2. P. Oakesianus. 



Submerged leaves with a proper blade. 



Submerged leaves of 2 kinds, lanceolate and oval or oblong. 

 Uppermost broadly oval or elliptic, lowest lanceolate. 



3. P. amplifolius. 

 Uppermost lanceolate and pellucid, lowest oblong and opaque. 



4. P. pulc/ier. 

 Submerged leaves all alike, capillary or linear-setaceous. 



i-nerved or nerveless. 25. P. Vaseyi. 



3-nerved 26. P. lateralis. 



Submerged leaves all alike, linear. 



Nearly the same breadth throughout, obtusely pointed, coarsely cellular- 

 reticulated in the middle. 5. P. Nuttallii, 

 Broader at base, acute, without cellular-reticulation. 



9. P. heterophyllus. 

 Submerged leaves all alike, lanceolate. 



Uppermost leaves petioled, lowest sessile. 6. P. alpinus. 



All the leaves petioled. 



Floating leaves large, broadly elliptic, rounded or subcordate at 

 base. ii. p, IlHnoensis. 



Floating leaves narrowly elliptic, tapering at base. 



7. P, lone kites. 

 Floating leaves mostly obovate or oblanceolate, tapering at base. 



8. P. Faxoni. 

 All the leaves sessile or subsessile. 



Fruit only 2 mm. long, obscurely 3-keeled. 



10. P. spathulaeformis* 

 Fruit 3 mm. long, distinctly 3-keeled. 12. P. Zizii. 



