Calyx short-campanulate or semiglobose, with no appendages at the sinuses (or a mere callous point). Petals none. Stamens 4, short. Capsule globular, indehiscent, 2-celled.--Submersed aquatic (sometimes terrestrial), rooting in the mud, with opposite linear leaves, and very small greenish flowers solitary in their axils. ("Didiplis means _two doubling_;" from δíς, _twice_, and διπλóος, _double_.) 1. D. lineàris, Raf. Leaves when submersed elongated, thin, closely sessile by a broad base, when emersed shorter and contracted at base; calyx with broad triangular lobes; style very short; capsules very small. (Ammannia Nuttallii, _Gray_.)--From Minn. and Wisc. to Tex., east to N. C. and Fla. 2. ROTÀLA, L. Calyx short-campanulate or semiglobose, with tooth-like appendages at the sinuses (abnormally, in our species). Petals 4 (in ours). Stamens 4, short. Capsule globular, 4-celled, septicidal, the valves (under a strong lens) transversely and densely striate. (Name a diminutive of _rota_, a wheel, from the whorled leaves of the original species.) 1. R. ramòsior, Koehne. Leaves tapering at base or into a short petiole, linear-oblanceolate or somewhat spatulate; flowers solitary (rarely 3) in the axils and sessile; accessory teeth of calyx as long as the lobes or shorter. (Ammannia humilis, _Michx._)--Low or wet ground, Mass. to Fla., west to Ind., Kan., and Tex.--With Ammannia-like habit, an exception in the genus. 3. AMMÁNNIA, Houston. Flowers in 3--many-flowered axillary cymes. Calyx globular or bell-shaped, 4-angled, 4-toothed, usually with a little horn-shaped appendage at each sinus. Petals 4 (purplish), small and deciduous, sometimes wanting. Stamens 4--8. Capsule globular, 2--4-celled, bursting irregularly.--Low and inconspicuous smooth herbs, with opposite narrow leaves, and small flowers in their axils, produced all summer. (Named after _Paul Ammann_, a German botanist anterior to Linnæus.) 1. A. coccínea, Rottb. Leaves linear-lanceolate (2--3´ long), with a broad auricled sessile base; cymes subsessile, dense; petals purplish; stamens more or less exserted; style usually slender; capsule included. (A. latifolia, _Gray_, Manual, not _L._)--N. J. to Fla., west to S. Ind., Kan., and Tex. The style varies much in length, sometimes in the same specimen. Apparently the more developed form of the southern A. latifolium, L., which, as limited by Koehne, has apetalous flowers, with included stamens and short style. 4. LÝTHRUM, L. LOOSESTRIFE. Calyx cylindrical, striate, 5--7-toothed, with as many little processes in the sinuses. Petals 5--7. Stamens as many as the petals or twice the number, inserted low down on the calyx, commonly nearly equal. Capsule oblong, 2-celled.--Slender herbs, with opposite or scattered mostly sessile leaves, and purple (rarely white) flowers; produced in summer. (Name from λύθρον, _blood_; perhaps from the styptic properties of some species.)