Juncus mertensianus, Merten's rush

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Intermountain Flora
Stems crowded on a stout rhizome, terete, 1-2.5 dm tall; leaves all cauline, the lowermost ones reduced, the others with terete, hollow, septatenodulose blade 4-15 cm long and 1-2 mm thick, the hyaline margins of the sheath expanded above and prolonged into a pair of evident auricles ca 1-1.5 mm long; inflorescence a single terminal head (seldom 2 heads) 8-12 mm thick, often shortly surpassed by the lower bract; flowers numerous, axillary to hyaline-scarious bracts but not prophyllate; tepals shiny, dark purple-brown or blackish-purple, thin and soft, the outer ones gradually acuminate, (2.5) 3-3.5 (4) mm long, the inner similar or often a little shorter and less sharply pointed; stamens 6; anthers mostly 0.5-1.0 mm long, shorter than the filaments; capsule triquetrous, somewhat expanded upwards, obtuse to broadly rounded or subtruncate or even slightly retuse beneath the stylar apiculus, slighty to evidently shorter than the perianth, 1-celled, the placentae only slightly intruded; seeds apiculate, ca 0.5 mm long.

Cronquist, A., Holmgren, A.H., Holmgren, N.H., Reveal, J.L., and Holmgren, P.K. (1977) "Intermountain Flora" pg. 60, vol. 6, The New York Botanical Garden New York.