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Potamogeton zosteriformis — flat-stem pondweed

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New England distribution

Adapted from BONAP data

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North America distribution

Adapted from BONAP data

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Facts

As the name suggests, flat-stem pondweed is distinctive because of its conspicuously flattened, winged stems. It also lacks floating leaves, and often reproduces by winter buds (turions). Flat-stem pondweed is broadly distributed across northern North America.

Habitat

Lacustrine (in lakes or ponds), riverine (in rivers or streams)

Characteristics

Habitat
aquatic
New England state
  • Connecticut
  • Maine
  • Massachusetts
  • New Hampshire
  • Vermont
Leaf position
the leaves are all submerged underwater
Leaf arrangement
  • alternate: there is one leaf per node along the stem
  • whorled: there are three or more leaves per node along the stem
Leaf blade length
100–200 mm
Petal or sepal number
there are four petals, sepals, or tepals in the flower
Specific leaf type
the leaf is not divided, rather the blade is made up of one segment
Floating leaf shape
NA
Underwater leaf blade width
2–5 mm
Fruit type (general)
  • the fruit is dry but does not split open when ripe
  • the fruit is fleshy
Underwater leaf length
100–200 mm
Show all characteristics

  • Clonal plantlets

    Turion length
    40–75 mm

  • Flowers

    Carpels fused
    the carpel is solitary or (if 2 or more) the carpels are not fused to one another
    Flower lower lip length
    0 mm
    Flower number
    12–15
    Flower position
    the flowers are above the surface of the water
    Flower symmetry
    there are two or more ways to evenly divide the flower (the flower is radially symmetrical)
    Inflorescence length
    15–30 mm
    Inflorescence type
    the inflorescence is a spike (a long unbranched stem with flowers along it that lack stalks)
    Length of peduncle
    20–50 mm
    Nectar spur
    NA
    Number of carpels
    4
    Ovary position
    the sepals and/or petals are attached below the ovary
    Palate on corolla
    no
    Petal and sepal arrangement
    the flower includes only one cycle of petals or sepals
    Petal appearance
    the petals are green and/or leafy in texture
    Petal fringed edges
    the petals are not fringed
    Petal fusion
    the perianth parts are separate
    Petal hairs on inner/upper surface
    there are no hairs on the inner/upper petal surface
    Petal length
    2–2.6 mm
    Petal number
    4
    Petal or sepal number
    there are four petals, sepals, or tepals in the flower
    Pistil number
    4
    Sepal appearance
    NA
    Sepal length
    0 mm
    Sepal number
    0
    Sepals fused only to sepals
    NA
    Spur length
    0 mm
    Stamen number
    4
    Stamen position relative to petals
    NA
    Stamens fused to petals
    the stamens are fused near the bases of the petals or tepals
    Style number
    4

  • Fruits or seeds

    Fruit beak length
    0.6–1 mm
    Fruit length
    4–4.5 mm
    Fruit type (general)
    • the fruit is dry but does not split open when ripe
    • the fruit is fleshy
    Fruit type (specific)
    • the fruit is a drupe (fleshy, with a firm inner ovary wall that encloses a single seed)
    • the fruit is an achene (dry, usually 1-seeded, does not separate or split open at maturity)
    Fruit width
    3–3.5 mm

  • Glands or sap

    Oil glands on nodes
    • at least some of the nodes have pairs of oil glands
    • none of the nodes have oil glands
    Sap
    the sap is clear and watery

  • Growth form

    Lifespan
    the plant lives more than two years
    Root septa
    the roots do not have transverse septa
    Roots floating in water
    there are no clusters of roots floating in the water
    Turions
    the plant has turions
    Underground organs
    there are only slender roots on the plant

  • Leaves

    Bract position (Sparganium)
    NA
    Bract relative length
    At least 3854 mm
    Bracts
    neither the flowers nor their pedicels have bracts
    Floating leaf basal lobes
    NA
    Floating leaf blade width
    0 mm
    Floating leaf length
    0 mm
    Floating leaf shape
    NA
    Floating leaf tip
    NA
    Floral bract form
    NA
    Floral bract length
    0 mm
    Leaf arrangement
    • alternate: there is one leaf per node along the stem
    • whorled: there are three or more leaves per node along the stem
    Leaf blade length
    100–200 mm
    Leaf blade veins
    the lateral veins are parallel or slightly arched in the direction of the tip
    Leaf blade width
    2–5 mm
    Leaf position
    the leaves are all submerged underwater
    Leaf special features
    none of the mentioned special features are present
    Leaf-like branch segments
    0
    Leaf-like branch shape
    NA
    Specific leaf type
    the leaf is not divided, rather the blade is made up of one segment
    Staminate bract edge (Myriophyllum)
    NA
    Stipule appearance
    the stipules are firm or fibrous, and colored white, green or brown
    Stipule fused to leaf
    the stipules are not attached to the leaf blade at all
    Stipules
    the plant has stipules
    Stipules fused around stem
    the stipules do not forma closed tube around the stem
    Trap-bladder length
    0 mm
    Underwater leaf air passage number
    At least 1439
    Underwater leaf air passage relative width
    At least 4336
    Underwater leaf air passage row number
    0
    Underwater leaf blade edges
    the underwater leaf has smooth edges, without teeth
    Underwater leaf blade shape
    the underwater leaf blade is linear (very narrow with more or less parallel sides)
    Underwater leaf blade veins
    15–35
    Underwater leaf blade width
    2–5 mm
    Underwater leaf length
    100–200 mm
    Underwater leaf stalk
    no
    Underwater leaf stalk length
    0 mm
    Underwater leaf tip shape
    the tip of the underwater leaf is acuminate (tapers to a long, thin point)
    Veins in floating leaf
    0

  • Place

    Habitat
    aquatic
    New England state
    • Connecticut
    • Maine
    • Massachusetts
    • New Hampshire
    • Vermont
    Specific habitat
    • in lakes or ponds
    • in rivers or streams

  • Stem, shoot, branch

    Flowering stem growth form
    the flowering stem is upright

Wetland status

Occurs only in wetlands. (Wetland indicator code: OBL)

New England distribution and conservation status

Distribution

Connecticut
present
Maine
present
Massachusetts
present
New Hampshire
present
Rhode Island
absent
Vermont
present

Conservation status

Exact status definitions can vary from state to state. For details, please check with your state.

Massachusetts
widespread (S-rank: S5)
New Hampshire
extremely rare (S-rank: S1), endangered (code: E)

Native to North America?

Yes

Sometimes confused with

Potamogeton epihydrus:
floating leaves usually produced on some plants and submersed leaves with 3-13 veins, with prominent bands of lacunae on each side of the midrib (vs. P. zosteriformis, with floating leaves always absent and submersed leaves with 15-25 veins, lacking lacunae).

Synonyms

  • Potamogeton zosterifolius Schumacher ssp. zosteriformis (Fern.) Hultén

Family

Potamogetonaceae

Genus

Potamogeton

Help

From the dichotomous key of Flora Novae Angliae

28.  Potamogeton zosteriformis Fern. N

flat-stem pondweed. Potamogeton zosterifolius Schumacher ssp. zosteriformis (Fern.) Hultén 
• CT, MA, ME, NH, VT. Shallow, still or slow-moving, slightly acidic to basic water of lakes and rivers.

26×28. Potamogeton strictifolius × Potamogeton zosteriformis → Potamogeton ×‌haynesii Hellquist & Crow is a rare pondweed hybrid known from VT. It closely resembles P. strictifolius and, like that species, has light-colored, fibrous stipules and usually lacks lacunar bands on leaf blades. The hybrid also has similar winter buds to those of P. strictifolius. The hybrid shows leaf blades with 
7–21 veins and rarely produces fruit. Potamogeton strictifolius has leaf blades with 3–5 (–7) veins and routinely produces fruits.