The single Western Australian species M. chamaelea was transferred to that genus from Sebastiania by Esser (1999).
Habit and leaf form. Shrubs (subshrubs), or herbs; evergreen; laticiferous (latex milky). Plants non-succulent; unarmed. Perennial. Leaves cauline. Plants with neither basal nor terminal concentrations of leaves; to 2 m high; tap rooted. Mesophytic, or xerophytic. Leaves small to medium-sized; alternate; spiral; ‘herbaceous’; petiolate; non-sheathing; not gland-dotted; simple. Leaf blades entire; flat; linear, or ovate, or elliptic; pinnately veined; cordate (subcordate), or attenuate at the base. Mature leaf blades glabrous to pilose; adaxially glabrous; abaxially glabrous to pilose. Leaves with stipules. Stipules intrapetiolar; free of one another; leafy (in S. chamaelea small, triangular, ciliate); caducous. Leaf blade margins serrate (minutely serrulate with glandular teeth, appressed in the Australian species M. chamaelea); flat. Leaves without a persistent basal meristem. Leaf anatomy. Hairs present; glandular hairs present. Unicellular hairs absent. Branched hairs absent. Urticating hairs absent. Extra-floral nectaries absent. Stem anatomy. Nodes tri-lacunar, or unilacunar. Secondary thickening developing from a conventional cambial ring, or anomalous; from a single cambial ring.
Reproductive type, pollination. Fertile flowers functionally male, or functionally female. Unisexual flowers present. Plants monoecious. The unisexual flowers aggregated in different parts of the same inflorescence (the male flowers in clusters along the axis and the female flowers solitary or a few at the base of the axis). Female flowers aggregated in ‘inflorescences’; without staminodes. Male flowers aggregated in ‘inflorescences’; without pistillodes. Entomophilous.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’. Inflorescence many-flowered. Flowers in spikes, or in racemes (or thyrses). Inflorescences simple; terminal, or leaf-opposed; ascending. Flowers subsessile (male flowers), or sessile (female flowers); bracteate; ebracteolate; minute to small; regular; 3 merous. Floral receptacle with neither androphore nor gynophore. Free hypanthium absent. Hypogynous disk absent. Perianth sepaline; 3; 1 -whorled. Calyx present; 3; 1 -whorled; polysepalous; hairy (in the Australian species; glabrous in others); valvate; persistent. Sepals obovate (to spathulate; male flowers), or triangular (female flowers). Corolla absent. Androecium present, or absent (female flowers). Fertile stamens present, or absent (female flowers). Androecial members definite in number. Androecium 3. Androecial members free of the perianth; all equal; free of one another; 1 -whorled. Androecium exclusively of fertile stamens. Stamens 3; all more or less similar in shape; isomerous with the perianth; erect in bud, or inflexed in bud. Filaments glabrous. Anthers all alike; basifixed; dehiscing via longitudinal slits; extrorse, or latrorse; bisporangiate, or tetrasporangiate. Fertile gynoecium present, or absent (male flowers). Gynoecium 3 carpelled. The pistil 3 celled. Carpels isomerous with the perianth. Gynoecium syncarpous; synovarious, or synstylovarious (depending on interpretation of the partially fused styles); superior. Ovary plurilocular; 3 locular. Gynoecium non-stylate (usually, the 3 free elongated stigmas attached directly to the ovary), or stylate (very shortly joined). Styles 1 (when present); apical; persistent; hairless. Stigmas 3; 1 - lobed; dry type; papillate, or non-papillate; Group II type. Placentation axile, or apical. Ovules 1 per locule; pendulous; epitropous; with ventral raphe, or with dorsal raphe; arillate; orthotropous, or anatropous, or hemianatropous.
Fruit and seed features. Fruit non-fleshy; orange, or brown; hairy, or not hairy (but with 6 rows of spine-like appendages growing from the surface); a schizocarp (with a central persistent axis; the capsule of M. chamaelea smooth except for 1-several conical processes on the abaxial surface of each mericarp). Mericarps 3; comprising nutlets. Fruit 3 celled; 3 locular; elastically dehiscent (schizocarpic capsules often splitting elastically), or passively dehiscent; 1–3 seeded. Seeds 1 per locule; 1 per mericarp. Seeds ovoid or obloid; endospermic. Endosperm oily. Seeds arillate. Cotyledons 2 (usually wider than the radicle). Embryo straight, or curved. Seedling. Germination phanerocotylar, or cryptocotylar.
Physiology, biochemistry. Mustard-oils present, or absent.
Geography, cytology, number of species. World distribution: mainly tropical South America but extending north to Mexico, tropical Africa with one species extending through Asia to northern Australia and the western islands of Indonesia. Native of Australia. Not endemic to Australia. Australian states and territories: Western Australia, Northern Territory, and Queensland. Northern Botanical Province and Eremaean Botanical Province. Microstachys chamaelea, the only species extending outside Africa in the Old World, is largely a shoreline or river bank plant. A genus of 14 species; 1 species in Western Australia; 0 endemic to Western Australia.
Additional characters Distinguishing features of Microstachys within the tribe Hippomaneae: inflorescences mostly opposite the leaves; the pistillate flowers often separated from the staminate part of the thyrse, i.e. situated as different parts of the stem; the multiple appendages of the always sessile pistillate flowers and fruits; the peculiar morphology of the central columella of the fruit with regularly paralllel margins; and the cylinbdircal seeds with a large stipitate caruncle (Esser 1999). Perianth of male flowers sepaline; 3. Perianth of female flowers sepaline; 3.
Etymology. From the Greek micro (small) and stachys (spike), presumably referring to an aspect of the inflorescence.