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Key to Plantago

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1 Leaves cauline, opposite; spikes on peduncles from the leaf axils; [subgenus Psyllium, section Psyllium]
1 Leaves basal; spikes on scapes from the base of the plant.
  2 Leaves pinnatifid; [subgenus Coronopus, section Coronopus]
  2 Leaves unlobed and either entire or toothed (if so, obscurely so in most species).
    3 Leaf venation pinnate, some major veins departing from the midvein well above the leaf base; summer and winter leaves dimorphic, the winter leaves lanceolate (typically submersed), the summer leaves ovate or cordate (emersed except in floods); plants perennial from a thick, fleshy rootstock that is typically 3-8 cm wide near its summit, with a cavity below (like an inverted bowl), and with 3-10 fleshy roots 3-15 mm thick descending or spreading from the bowl rim; capsule 2-4-seeded; scapes hollow and terete; [aquatic or semi-aquatic]; [subgenus Plantago, section Heptaneuron]
    3 Leaf venation parallel, with all of the major veins separating at the base of the leaf blade; leaves not dimorphic; plants either perennial from thin, fibrous roots or an erect caudex, or annual from a small taproot; capsule 2-30-seeded; scapes solid, terete or 5-angled; [terrestrial or wetland].
      4 Leaves ovate to broadly lanceolate or broadly oblanceolate, 1-10× as long as wide, distinctly broadened upward from a petiolar base, the larger leaves > 1 cm wide.
        5 Leaves broadly ovate-elliptic, the blades 1-3× as long as wide, distinctly petiolate; scapes solid and terete.
          6 Bracts and sepals flat or slightly keeled; seeds 2-4 per capsule; primary roots thickened, tuberous near junction with the stem and leaves; spikes grayish or whitish, shiny, densely flowered such that the corolla lobes of adjacent flowers often overlap; [subgenus Plantago, section Lamprosantha]
          6 Bracts and sepals conspicuously keeled; seeds 4-22 (-34) per capsule; primary roots all slender and filamentous; spikes brownish or greenish, dull, sparsely to moderately densely flowered, such that the corolla lobes of adjacent flowers do not overlap.
             7 Fruit (3-) 4-6 mm long, dehiscent below the middle (the terminal portion about 2× as long as the basal); sepals narrowly elliptic, 2-4× as long as wide, mostly acute; petioles usually purple and glabrous at the base; well-developed leaves with 5-7 major veins; seeds 4-10 per capsule, each 1.5-2.0 (-2.5) mm long; [subgenus Plantago, section Pacifica]
             7 Fruit (2-) 2.5-4 mm long, dehiscent near the middle (the terminal portion about as long as the basal); sepals broadly ovate, ca. 1.5× as long as wide, mostly obtuse; petioles usually green and pubescent at the base; well-developed leaves with 3-5 major veins; seeds (4-) 6-22 (-34) per capsule, each 0.7-1.7 mm long; [subgenus Plantago, section Plantago].
               8 Fruit rounded to pointed at apex; seeds (4-) 6-11 (-13) per fruit; leaf blades membranaceous to fleshy-herbaceous, glabrous or pubescent; petioles elongate and slender; [widespread in our region]
               8 Fruit broadly rounded at apex; seeds 14-22 (-34) per fruit; leaf blades subcoriaceous-fleshy, usually pubescent; petioles short and broad; [estuarine shores, DE northwards]
        5 Leaves mostly broadly oblanceolate, broadly lanceolate, or spatulate, (3-) 4-10× as long as wide, attenuate to the only somewhat petiolar base; scapes solid and 5-angled, or hollow (to solid) and terete.
                 9 Bracts and calyx glabrous; perennial, flowering Apr-Nov.
                   10 Spikes very densely flowered, the rachis hidden; scape 5-angled; [widespread weedy exotic]; [subgenus Plantago, section Lancifolia]
                   10 Spikes loosely flowered, the rachis visible its entire length; scape terete; [rare native of Coastal Plain pinelands and adjacent fire-plow lines and ditches]; [subgenus Plantago, section Pacifica]
                 9 Bracts and calyx pubescent, at least on the keels; annual or perennial; flowering late Mar-Aug; [subgenus Plantago, section Virginica].
                     11 Perennial from a caudex; flowering Jun-Aug; upper leaf surface floccose; [local adventive, Panhandle FL]
                     11 Ephemeral annual from a taproot, flowering late Mar-Jun, and then senescing; upper leaf surface glabrate or pilose; collectively widespread and common].
                       12 Mature seeds 2.5-3 mm long, dark red, nearly flat on both sides; sepals 2.7-3.6 mm long, with an excurrent midrib; leaves typically toothed; [rare adventive in western part of our area]
                       12 Mature seeds 1.5-2 mm long, yellowish, tan, or brown, concave on one side, convex on the other; sepals 1.5-2.4 mm long, obtuse to rounded; leaves entire; [common in our area]
      4 Leaves linear or narrowly lanceolate, (8-) 10-25× as long as wide, only slightly (if at all) broadened upward, the base not petiolar, the leaves typically < 1 cm wide.
                          13 Leaves fleshy; corolla tube pubescent on its outer surface; plants perennial; [of sea beaches, in our region from e. VA northwards]; [subgenus Coronopus, section Maritima]
                          13 Leaves herbaceous; corolla tube glabrous on its outer surface; plants annual; [of various habitats, collectively widespread].
                            14 Seeds (3-) 4-25 (-30) per capsule; corollas radially symmetrical, the lobes 0.5-1 mm long; stamens 2; [subgenus Plantago, section Micropsyllium]
                              15 Seeds 0.5-0.8 mm long; capsules with 10-25 (-30) seeds
                              15 Seeds 0.8-2.5 mm long; capsules with (3-) 4-9 (-12) seeds.
                                16 Corolla lobes spreading to even reflexed; capsules with (3-) 4-9 (-12) seeds; seeds 1.5-2.5 mm long
                                16 Corolla lobes erect, connivent into a beak; capsules with 4 seeds; seeds 0.8-1.3 mm long
                            14 Seeds 2 per capsule; corollas bilaterally or radially symmetrical, the lobes 1.3-3.6 mm long; stamens 4; [subgenus Psyllium, section Gnaphaloides].
                                  17 Corollas radially symmetrical; bases of the corolla lobes obtuse or slightly cordate; [c. and nc. TX and westwards]
                                  17 Corollas bilaterally symmetrical; seeds 3.2-3.8 mm long; [collectively widespread].
                                    18 Stems (below the lowermost flowers) 20-40 cm long; bracts subtending the flowers either 2-3 mm long or 10-30 mm long.
                                       19 Bracts subtending the flowers 10-30 mm long; uppermost corolla lobe 1.4-2.3 mm long
                                       19 Bracts subtending the flowers 2-3 mm long; uppermost corolla lobe 2.4-3.0 mm long
                                    18 Stems 0-15 mm long; bracts 1.5-10 mm long; [mainly OK and TX westwards, rarely adventives eastwards].
                                         20 Leaves 4-10 mm wide, the margins toothed, the teeth to 4 mm long (rarely entire); corolla lobes 2.2-2.5 mm long
                                         20 Leaves 1-4 mm wide, the margins entire (rarely toothed); corolla lobes 1.6-2.1 mm long

Key C3: rooted aquatics with basal and simple, broad leaves

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1 Leaves peltate.
  2 Leaf blades oval in shape, ca. 1.5-2× as long as wide, at maturity floating on the water’s surface; underwater portions of fresh plant coated in transparent mucilage
  2 Leaf blades orbicular in shape, ca. 1× as long as wide, at maturity floating on the water’s surface, emersed, or submersed; underwater portions of fresh plant not mucilaginous (though possibly with green algae, etc.).
    3 Leaves small, < 8 cm in diameter, at maturity emersed or submersed
    3 Leaves large, > 20 cm in diameter, at maturity floating on the water’s surface or emersed
1 Leaves not peltate, the petiole attached at a cuneate, cordate, or sagittate base.
      4 Leaf blades cuneate, rounded, or truncate at the base.
        5 Basal leaves lanceolate, 3-6 mm long
        5 Basal leaves variously shaped, > 10 mm long.
          6 Main veins palmate from the leaf base and also diverging from the midvein, the secondary and tertiary veins then reticulating; inflorescence a spike; [Eudicots]
          6 Main veins either parallel or palmate from the leaf base with cross-veins at nearly right angles to the main veins; inflorescence either a diffuse raceme or panicle of white flowers, or a linear spadix of tightly packed golden-yellow flowers; [Monocots].
             7 Inflorescence diffuse, a raceme or panicle with whorled branches or pedicels, the flowers widely spaced and white; leaves green, “wettable”
             7 Inflorescence tightly packed with flowers, an elongate, golden yellow spadix; leaves blue-green, “unwettable”
      4 Leaf blades cordate or sagittate at the base.
               8 Leaf blades sagittate, the two lobes distinctly acute; leaf apex acute; leaf blade 1.3-3× as long as wide
               8 Leaf blades cordate, the two lobes rounded or sub-acute; leaf apex rounded or apiculate; leaf blade 0.8-8× as long as wide.
                 9 Leaf blade margins serrate, dentate, and/or incised
                 9 Leaf blade margins entire or obscurely crenate.
                   10 Flowers 4-5-merous (sepals 4-5, petals 4-5, stamens 4-5); [Eudicots]
                     11 Leaves emersed or submersed, on stout, stiff petioles (the submersed and winter leaves cuneate-based and lanceolate); flowers 4-merous (sepals 4, petals 4, stamens 4); inflorescence a spike
                     11 Leaves floating, on slender, flexuous petioles (all leaves cordate-based); flowers 5-merous (sepals 5, petals 5, stamens 5); inflorescence an umbel
                   10 Flowers 3-merous or many (>5-) -merous; [Basal Angiosperms or Monocots].
                       12 Perianth parts numerous (usually showing differentiation into sepals and petals, though often with some intergradation), borne in a spiral; stamens numerous; leaves usually > 10 cm long or > 10 cm wide, or both (a few northern species of Nymphaea with leaves as small as 2.5 cm × 2.5 cm); [Basal Angiosperms]
                       12 Perianth parts 3-6 (either differentiated or not into sepals and petals); stamens either 3, or 9-12 (-18), or >20; leaves 1-10 cm long, 1-10 cm wide; [Monocots].
                          13 Perianth strongly differentiated, the 3 sepals green, leathery, and persistent, the 3 petals yellow or white, delicate, deciduous; stamens 20 or more
                          13 Perianth not strongly differentiated, the sepals petaloid and similar in texture and color to the petals; stamens 10 or fewer.
                            14 Flowers unisexual, white, the sepals and petals separate; stamens 3, 6, 9, 12, 15, or 18; leaves either with a central area of spongiform cells (most easily seen on the lower leaf surface) (Limnobium), or without spongiform cells (Ottelia)
                            14 Flowers bisexual, white to blue, the perianth segments united below into a perianth tube 3-45 mm long; stamens 3; leaves lacking a central area of spongiform cells