Flower:

Whorls of tiny flowers at the leaf nodes all along the emersed part of the stem, with a single flower in each leaf axil. Flowers are petal-less, green to purplish, oval, about 1 mm long, either male with a single purplish stamen, female with a single thread-like style, or perfect with both a style and a stamen.

Leaves and stems:

Plants are usually at least partially submersed, though may be wholly emersed or less often wholly submersed. Emersed leaves are lance-linear, up to 1 inch long, fairly stiff, whorled around the stem in groups of 8 to 12, toothless, hairless, pointed at the tip, stalkless, usually widely spreading and often slightly curved. Submersed leaves are similar in shape but are limp, much longer, typically with 12 to 16 in a whorl and the whorls densely crowded on the stem.

Stems are stout, hollow, unbranched, hairless, often prostrate from the base and rooting at the nodes. Emersed stems become erect and stiff while submersed stems are limp. Plants can grow in water up to 10 feet deep, and form large colonies from creeping rhizomes, the older part of the stem dying off after new growth establishes. Submersed plants spread vegetatively, while emersed plants also reproduce by seed.

Fruit:

Fruit is a 1-seeded oval capsule 1.5 to 3 mm long. Seeds are oval-elliptic and smooth.

Notes:

Mare's Tail is a circumpolar species present in both North America and in northern Europe and Asia, found in the slow moving or stagnant waters of rivers, lakes, ditches and marshes, and on receding shores in sandy, gravelly or mucky soil. At first glance land-locked plants may resemble a Clubmoss, which is a non-flowering plant that reproduces by spores. Submersed plants vaguely resemble some other aquatics with narrow, whorled leaves, but Mare's Tail is distinguished by the stout, hollow, unbranched stems arising from a rhizome; simple, unlobed, limp leaves in whorls of up to 16; the whorls crowded on the stem. If there are emergent leaves, the shorter, stiffer leaves and whorls of flowers or fruits further distinguish Mare's Tail.