1	Stems floating or slightly prostrate in water; leaves usually floating, flattened		(2)
+	Stems erect; leaves aerial, not flattened		(5)
2 (1)	Stems stout, 1.3-1.5 m tall; leaves ca. 10 mm wide; male heads 8-10.		8 S. yunnanense
+	Stems slender, less than 70 cm tall; leaves 2-4 mm wide; male heads 1-4		(3)
3 (2)	Styles absent or inconspicuous; fruit broadly obovoid.		11 S. hyperboreum
+	Styles conspicuous; fruit fusiform, elliptic, or lanceolate		(4)
4 (3)	Stems floating, more than 20 cm; male heads 2-4.		9 S. angustifolium
+	Stems slightly prostate or rarely erect, less than 20 cm; male heads 1 or 2.		10 S. natans
5 (1)	Inflorescences paniculate, with conspicuous lateral branches with male and female heads; ovary sessile		(6)
+	Inflorescences racemose or spicate, degenerated into 1 female head or completely degenerated; ovary with short stalk		(8)
6 (5)	Panicles with 3-7 lateral branches; stigmas 3-4 mm.		1 S. stoloniferum
+	Panicles with 1(or 2) lateral branches; stigmas less than 2 mm		(7)
7 (6)	Leaves 2-3 mm wide; main axis and lateral branch of panicles not zigzag; styles ca. 0.5 mm.		2 S. subglobosum
+	Leaves 4-5 mm wide; main axis and lateral branch of panicles usually zigzag; styles 1-1.5 mm.		3 S. limosum
8 (5)	Inflorescences spicate; perianth segments lobate to parted.		7 S. confertum
+	Inflorescences racemose; perianth segments serrate-lobate to lobate		(9)
9 (8)	Axis of racemes curved; ovary attenuate toward base and forming an inconspicuous stalk.		4 S. fallax
+	Axis of racemes not curved; ovary with a conspicuous stalk		(10)
10 (9)	Male heads 4-8, distinctly separate from female ones; stigmas 1.5-1.8 mm.		5 S. emersum
+	Male heads 1-3, not separate from female ones; stigmas ca. 0.5 mm.		6 S. glomeratum