350 94. POTAMOGETONACE^E. 



inclosing a hard nut, or a dry nut, not bursting, 1-seeded. Albu- 

 men 0. Embryo with a thin skin having a lateral cleft. 



1. POTAMOGETON. Fl, perfect. Perianth 4-parted. Anth. 

 4, sessile, opposite to the divisions of the perianth. Ovaries 

 4. Styles 0. Drupes 4, sessile. Fl. sessile, spiked. 



2. RUPPIA. Fl. perfect. Perianth 0. Stam. 2, the cells 

 considerably separated ; filaments very short, scalelike. 

 Ovaries 4. Styles 0. Nuts 4, with long stalks. Fl. about 

 2 together. 



3. ZANNICHELLIA. Fl. imperfect, axillary. Barren with 1 

 stam., and no perianth. Fertile with a bellshaped perianth, 

 persistent style, and peltate stigma. Nuts 3 5 or more, 

 very shortly stalked. 



1. POTAMOGETON Linn. Pondweed. 



* L. alternate, floral I. floating and sometimes opposite ; stipules 

 free. 



1. P. natans (L.) ; 1. all stalked, upper coriaceous floating 

 ovate or elliptical plicate at the base, lower leafless petioles 

 linear or setaceous, fr. (large) rounded on the back when fresh 

 keeled when dry, peduncles equal. E. B. 1822. R. vii. 50. A 

 creeping rhizome. Lowermost 1. (phyllodes) often quite seta- 

 ceous ; upper 1. move or less cordate at the base, when pressed 

 flat a ridge is formed on each side of the base, jointed to their 

 stalks a little below the limb. Petioles plane-concave above. 

 Sep. stalked, roundly rhomboidal. Anth. -cells not parallel. Fr. 

 greenish, H 2 lin. long. Ponds, ditches, and slow streams. 

 P. VI. VII. 



2. P. polygonifolius (Pourr.) ; 1. all stalked, upper subcori- 

 aceous floating oblong-elliptical subcuspidate no leafless petioles, 

 lower 1. linear-lanceolate, fr, minute obtuse and rounded on the 

 back, peduncles equal. P. oblongus Viv. E. B. S. 2849. 

 St. creeping below. Petioles longer than leaves, plane-con- 

 cave above. Lower 1. often very narrow. Spikes rather short 

 and irregular. Sep. transversely elliptical, stalked. Fr. red- 

 dish. 1 lin. long, having a faint keel and rather obscure lateral 

 ridges when dry. Ditches, small streams, ponds. P. VII. 



3. P. plantagineus (Ducr.) ; I. all shortly stalked membranous 

 and pellucid blunt not cuspidate nor plicate, upper elliptical, 

 lower 1. oblong, fr. minute rounded on the back when fresh 

 keeled when dry, peduncles equal. E. B. S. 2848. R. vii. 45. 

 St. creeping below, branched, sometimes throwing out lotig 

 stoles from its upper axils. L. all beautifully transparent and 



