3. Potamogeton diversifolius Raf. (waterthread pondweed)

Pl. 191 e, f; Map 772

Plants with slender rhizomes. Stems branched, circular in cross‑section or nearly so, lacking purplish black spots or glands at the nodes. Submerged leaves with the leaf blades flat, with 1(3) main veins, 1–10 cm long, 1.0–1.5 mm wide, linear, the tip pointed to acuminate, the base tapering, sessile, the margins entire, the stipules 0.2–1.8 cm long, fused to the leaf base in the basal 1/3–1/2. Floating leaves often present, herbaceous, with leaf blades 0.5–4.0 cm long, 0.2–2.0 cm wide, narrowly elliptic to lanceolate or suborbicular, the tip rounded or pointed, the base rounded or tapering to a petiole 0.2–4.0 cm long, the stipular sheath 0.2–2.5 cm long, free from the leaf base and usually clasping the stem. Spikes of upper leaves emergent, 0.3–3.0 cm long, the stalks 0.3–3.0 cm long, slightly thickened near the tip, grading into the submerged spikes of the middle and lower leaves, which are 0.2–0.5 mm long, the stalks 0.1–1 cm long, recurved at fruiting, slightly thickened near the tip. Fruits 1–2 mm long including the minute beak, orbicular or nearly so, the sides flattened or sometimes shallowly concave, each usually with a low ridge or line of teeth, the back with a sharp‑edged, sometimes irregular keel, lacking a toothlike basal appendage. May–October.