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Potamogeton
See list of 28 species in this genusSpecies of Potamogeton produce one or both of two types of leaves—submersed and floating. All species of Potamogeton produce submersed leaves, which are very thin, translucent, and generally occur below the surface of the water (unless stranded). Floating leaves are thick, opaque, and occur on the surface of the water during normal seasons. Sometimes submersed leaves appear absent due to their decay in the later season or mechanical damage (e.g., wave action). Oil glands, referred to as nodal glands, occur in many species (most common in those with sessile, submersed leaves). They appear as a pair of green- to gold-colored circular bumps at the nodes. They are sometimes difficult to detect and should be sought on several nodes to assess their presence/absence. Hybrid plants are infrequently collected but are most common among the broad-leaved species and in riverine communities. They are typically sterile and do not produce fruits (though they do flower). Potamogeton alpinus ×P. epihydrus was reported from ME by Angelo and Boufford (2000), but the specimens (at MASS!) have qualified (i.e., tentative) determinations and do not have morphologies that suggest the involvement of P. epihydrus (e.g., the lacunar bands of the leaf blade are too few). Potamogeton ×nericius Hagstr. was reported from ME by Angelo and Boufford (2000), but the only collection observed (at MASS!) has a qualified (i.e., tentative) determination and appears equivocal. Potamogeton illinoensis ×P. perfoliatus was reported from MA and P. ×prussicus Hagstr. was reported from VT by Angelo and Boufford (2000), but specimens are unknown.
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1a. Submersed leaf blades narrower than 4 mm wide
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2a. Submersed leaf blades with the stipule basally fused to the blade, only the tip of the stipule distinct from the blade [Fig. 299]; drupe with an embryo coiled more than 1 full spiral [Fig. 289]
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3a. Submersed leaf blades 0.1–0.4 mm wide, the apex acute to long-tapering; fused portion of the stipule usually shorter than the distinct portion; apex of floating leaves acute
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3b. Submersed leaf blades 0.5–2 mm wide, the apex obtuse to acute; fused portion of the stipule usually equal to or longer than the distinct portion; apex of floating leaves obtuse
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2b. Submersed leaf blades with stipules that are distinct from the blade for their entire length [Fig. 287]; drupe with an embryo coiled 1 full spiral or less [Fig. 294]
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4a. Floating leaves, which are thicker and greatly expanded in width compared with the submersed leaves, produced and generally present on some of the plants in each colony [Fig. 296]
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5a. Floating leaf blades 6–15 mm long, 5- to 9-veined; submersed leaf blades 0.1–1 mm wide, thin, transparent, lacking lacunar cells; spikes (3–) 6–8 mm long; drupes 1.5–2.5 mm long
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5b. Floating leaf blades 15–120 mm long, 7- to 37-veined; submersed leaf blades 0.25–10 mm wide, either with abundant lacunar cells or with thick and nearly opaque texture; spikes 10–50 mm long; drupes (2–) 2.5–4.5 mm long
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6a. Submersed leaf blades (1–) 2–10 mm wide, thin, transparent, with prominent bands of lacunar cells 1–2 mm wide on each side of the midrib [Fig. 291]; drupes with abaxial and lateral keels (in part)
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6b. Submersed leaf blades 0.25–2 mm wide, thick and nearly opaque, petiole-like, lacking lacunar cells; drupes with only an abaxial keel
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7a. Submersed leaf blades 0.8–2 mm wide; floating leaf blades 5–10 ×2.5–6 cm, usually subcordate to rounded at the base [Fig. 296]; petioles pale at the apex; stipules 4–10 cm long; drupes 3.5–5 mm long, deeply wrinkled on each side, with an inconspicuous abaxial keel
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7b. Submersed leaf blades 0.25–1 mm wide; floating leaf blades 2.5–6 ×1–3 cm, cuneate to rounded at the base; petioles usually lacking a pale band at the apex; stipules 2.5–4 cm long; drupes 2–3.5 mm long, not wrinkled on the sides, with a prominent abaxial keel
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8a. Rhizome elongate; peduncles (3–) 5–25 cm long [Fig. 290]; leaf blades 0.1–0.5 mm wide, 1-veined
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8b. Rhizome scarcely developed (elongate in P. foliosus); peduncles 0.5–6 cm long [Fig. 292]; leaf blades 0.2–5 mm wide, 1- to 35-veined
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9a. Stems conspicuously compressed; leaf blades 15- to 25 (–35)-veined; spikes with 7–11 whorls of flowers; drupes 4–4.5 mm long (in part)
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9b. Stems terete or slightly compressed; leaf blades 1- to 13-veined; spikes with 1–5 whorls of flowers; drupes 1.5–3.5 mm long
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10a. Nodal glands absent (very rarely present); spikes 1.5–7 mm long, with 1 or 2 whorls of flowers [Fig. 292]; drupes with a conspicuous abaxial keel
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11a. Leaf blades acute at apex; drupes 1.4–2.3 (–2.7) mm long, with a single, wing-like and undulate dorsal keel to 0.4 mm tall; peduncles 3–11 (–37) mm long
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11b. Leaf blades bristle-tipped (rarely only apiculate or blunt) [Fig. 295]; drupes 2.3–4 mm long, with 3 keels, the dorsal keel ridge-like, to 0.2 mm tall; peduncles 6–14 mm long
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10b. Nodal glands present on at least some of the nodes [Fig. 288]; spikes 2–18 mm long, with 1–5 whorls of flowers; drupes without an abaxial keel (sometimes with a low keel in P. obtusifolius)
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12a. Stipules fibrous, white (brown in P. ogdenii)
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13a. Leaf blades obtuse to rounded at the apex, sometimes with an apiculus; winter buds with inner leaves oriented at a right angle to the outer leaves, the outer leaves corrugated at the base, the inner leaves modified into a fan-shaped structure [Fig. 293]
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13b. Leaf blades obtuse to attenuate at the apex; winter buds flattened, the inner and outer leaves oriented in the same plane, the outer leaves not corrugated at the base (rarely corrugated in P. strictifolius), the inner leaves undifferentiated or modified into a fusiform structure [Fig. 300]
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14a. Leaf blades with 3–5 (–7) veins, 0.6–2 mm wide, without lacunae; stipules usually white, the margins connate; drupes 1.9–2.1 mm long; inner leaves of winter bud undifferentiated
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14b. Leaf blades with 3–9 (–13) veins, 1.2–2.9 mm wide, with 0–3 rows of lacunae on each side of the midrib; stipules usually brown, convolute; drupes 2.5–3 mm long; inner leaves of winter bud commonly rolled into a hard, fusiform structure
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12b. Stipules delicate, not fibrous, green, brown, or white
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15a. Leaf blades 1–3.5 mm wide, obtuse to rounded at the apex, sometimes with an apiculus, commonly suffused with red; drupe 2.5–3.5 mm long, with a beak 0.6–0.7 mm long, the embryo coiled 1 full spiral
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15b. Leaf blades 0.2–2.5 mm wide, acute to obtuse at the apex, green; drupe 1.5–2.2 mm long, with a beak 0.1–0.6 mm long, the embryo coiled less than 1 full spiral
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16a. Leaf blades 0.2–0.7 mm wide, acute to long-acute at the apex, 1-veined, with a single row of lacunae on each side of the midvein; peduncles 10–35 mm long; drupes with beak centrally positioned on the edge of fruit; stipules convolute, with inrolled but distinct margins, attenuate at the apex, subherbaceous in texture; spikes 1–3 per plant, the whorls of fruit usually continuous
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16b. Leaf blades 0.2–2.5 mm wide, acute to obtuse (rarely apiculate) at the apex, usually 3- or 5-nerved, with 0–5 rows of lacunae on each side of the midvein; peduncles 5–62 mm long; drupe with beak centrally positioned on edge of fruit or positioned toward one margin; stipules convolute with distinct margins or connate with fused margins, in either case obtuse to acute at the apex, scarious to subherbaceous; spikes mostly 1–10 per plant, the whorls of fruit continuous or interrupted
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17a. Leaf blades with 0–2 rows of lacunae on each side of the midvein, often with none, acute at the apex (rarely apiculate); stipules with united margins for more than ½ their length, forming a tube surrounding the stem, scarious; spikes mostly terminal, 1–3 per plant, with peduncles 10–62 mm long, interrupted and with 2–4 separate whorls of fruits; drupes with beak positioned toward the margin of fruit (rarely centrally positioned on edge)
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17b. Leaf blades with 1–5 rows of lacunae on each side of the midvein, acute to obtuse at the apex; stipules convolute, flat or with inrolled but distinct margins, scarious to subherbaceous; spikes terminal and/or axillary, mostly 2–10 per plant, usually more than 3, with peduncles 5–30(–45) mm long, continuous and with 1–3 crowded whorls of fruits; drupes with beak centrally positioned on the edge of fruit (rarely positioned toward the margin)
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1b. Submersed leaf blades 4 mm wide or wider (those of the axillary shoots may be narrower in some species)
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18a. Stipules basally fused to the leaf blade, only the tip of the stipule distinct from the blade; leaf blades usually minutely serrulate, especially in the apical portion, with a pair of auricles at the base; well-formed inflorescences branched [Fig. 298]
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18b. Stipules of submersed leaves distinct from the leaf blade their entire length; leaf blades entire or toothed in P. crispus,
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19a. Leaf margins conspicuously serrulate; beak of drupe 2–3 mm long; turions commonly formed
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19b. Leaf margins entire; beak of drupe 0.3–0.9 mm; turions absent or rarely formed
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20a. Submersed leaf blades sessile and conspicuously cordate-clasping at the base [Fig. 297]; floating leaves not produced
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21a. Leaf blades strictly entire, (60–) 80–280 mm long, the apex cucullate and usually splitting when pressed; rhizome spotted with red-brown; stipules persistent; drupes 4–5.7 mm long, with a prominent, abaxial keel
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21b. Leaf blades provided with minute, caducous, 1-celled spicules along the margin, 9–130 mm long, the apex flat and not splitting when pressed; rhizome unspotted; stipules disintegrating and remaining as fibers or completely absent; drupes 1.6–4.2 mm long, with an inconspicuous keel or the keel absent
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22a. Leaf blades ovate to suborbicular, 9–76 (–97) mm long, delicately 7- to 15-veined; stipules disintegrating and absent on the lower part of the stem; drupes 1.6–3 mm long
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22b. Leaf blades narrow-ovate to narrow-lanceolate, (16–) 30–130 mm long, coarsely 13- to 21-veined [Fig. 297]; stipules disintegrating into persistent fibers; drupes 2.2–4.2 mm long
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20b. Submersed leaf blades sessile or petiolate, not cordate-clasping; floating leaves produced and generally present on some of the plants in each colony or not produced in P. zosteriformis
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23a. Stems red-brown- or black-spotted [Fig. 287]; floating leaf blades rounded to cordate at base; submersed leaf blades usually crisped and arcuate [Fig. 287]; drupes 5–6.7 mm long
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24a. Submersed leaf blades narrow-lanceolate to lanceolate, with 7–19 veins; floating leaf blades with 15–19 veins; stems black-spotted
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24b. Submersed leaf blades ovate to broad-oblanceolate, with 19–49 veins; floating leaf blades with 27–49 veins; stems red-brown-spotted
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23b. Stems and petioles unspotted; floating leaf blades cuneate to rounded at the base or absent altogether in P. zosteriformis; submersed leaf blades usually flat and not arcuate (sometimes crisped and/or arcuate in P. illinoensis); drupes 1.9–4.3 mm long
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25a. Stems conspicuously compressed; floating leaves not produced; submersed leaf blades 15- to 25 (–35)-veined (in part)
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25b. Stems terete to somewhat compressed; floating leaves produced, though not always present; submersed leaf blades 3- to 19 (–21)-veined
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26a. Stipules (1–) 3–9 cm long; submersed leaf blades with 7–19 (–21) veins, sessile or with petioles up to 13 cm long
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27a. Submersed leaves with petioles (2–) 5–13 cm long, the larger leaves with blades 10–35 mm wide and acute at the apex; drupes red to red-brown, the lateral keels with blunt to sharp teeth
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27b. Submersed leaves sessile or with petioles up to 4 cm long, the larger leaves with blades 20–45 mm wide and mucronate at apex; drupes gray-green to olive-green, the lateral keels +⁄– without teeth
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26b. Stipules 1–3 cm long; submersed leaf blades with 3–13 veins, sessile
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28a. Submersed leaves distichous, the blades ± linear, with prominent lacunar bands 1–2 mm wide on each side of the midrib; each lacunar band composed of 9–18 rows of cells (in part)
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28b. Submersed leaves ± spirally arranged, the blades linear to narrow-obovate, with inconspicuous lacunar bands up to 0.5 mm wide on each side of the midrib; each lacunar band composed of 0–4 rows of cells
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29a. Submersed leaf blades 4.5–18 (–25) cm long, strictly entire, red-tinged, especially in drying; stems unbranched; drupes (2.5–) 3–3.5 mm long, plump and turgid, the exocarp hard, the embryo coiled 1 full spiral
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29b. Submersed leaf blades (1.5–) 3.1–9.1 (–10) cm long, provided with minute, caducous, 1-celled spicules along the margin, green; stems sparingly to many-times branched; drupes 1.9–2.3 mm long, laterally compressed, the exocarp soft, the embryo coiled less than 1 full spiral
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Each photo represents one species in this genus.