Fruits and male flowers are often necessary for positive determination of these highly variable aquatics.

1. Sepals closely appressed; pedicels thick (the lower ca. 3-5 mm in diameter); leaves very broadly sagittate, much broader than long; lowermost flowers mostly bisexual.

S. montevidensis

1. Sepals of mature pistillate flowers reflexed; pedicels more slender; leaves various; lowermost flowers ordinarily pistillate.

2. Filaments pubescent with ± flattened or scale-like hairs; leaves not sagittate (or small basal lobes occasionally present in S. rigida).

3. Pistillate (lower) flowers sessile or nearly so; axis of the inflorescence strongly bent above the lowest flowers; achenes with beak ca. 1-1.5 mm long.

S. rigida

3. Pistillate and staminate flowers pediceled; axis of inflorescence straight; achenes with beak ca. 0.2-0.8 mm long.

4. Achenes with beak 0.4-0.6 mm long; anthers clearly shorter than the filaments.

S. cristata

4. Achenes with beak minute, scarcely discernable, ca. 0.2 mm long; anthers as long as or longer than filaments.

S. graminea

2. Filaments glabrous; leaves usually sagittate when emersed.

5. Mature achenes with a ± horizontal beak projecting to one side and attaining 0.5-1.5 (-2.3) mm in length; faces of achene not winged, at most with a visible resin duct; bracts 4-9 mm long (or up to 11 mm in some robust plants).

S. latifolia

5. Mature achenes with an erect or curved apical beak up to ca. 0.7 mm long; faces of achene sometimes with a low wing or ridge; bracts usually (8-) 10-16 mm long.

6. Achenes with beak ca. 0.4-0.7 mm long, appearing as a continuation of the ± straight margin of the achene, straight or curved outward or over apex of achene.

S. brevirostra

6. Achenes with a very tiny erect beak scarcely 0.2-0.3 mm long, terminating a swollen or convex margin of the achene and thus appearing to project from summit of the achene, definitely set in from the margin.

S. cuneata