Mitreolaceae Watson, informal
Habit and leaf form. Herbs (or subshrubs, in Schizacme); non-laticiferous and without coloured juice. Plants non-succulent; autotrophic. Annual, or perennial; with a basal aggregation of leaves, or with neither basal nor terminal aggregations of leaves. Leaves opposite (mostly), or whorled; of M. minima sometimes 3 per whorl (at the lower nodes); ‘herbaceous’, or leathery (?), or fleshy; imbricate, or not imbricate; petiolate to sessile, or perfoliate; connate (via the stipular sheath); simple. Lamina entire; one-veined, or pinnately veined, or palmately veined. Leaves stipulate to exstipulate. Stipules interpetiolar (the opposite petioles connected across the node by recognisable but small and joined stipules or a stipular membrane); with colleters, or without colleters (?).
Leaf anatomy. Hairs present, or absent; eglandular. Complex hairs absent.
Stem anatomy. Young stems cylindrical, or oval in section. Internal phloem present. Secondary thickening anomalous; from a single cambial ring. ‘Included’ phloem present. Vessel end-walls scalariform, or simple.
Reproductive type, pollination. Plants hermaphrodite, or gynomonoecious; homostylous.
Inflorescence, floral, fruit and seed morphology. Flowers solitary, or aggregated in ‘inflorescences’; terminal, or axillary; in cymes, or in racemes, or in umbels. The ultimate inflorescence unit cymose. Inflorescences scapiflorous, or not scapiflorous; terminal and axillary, or axillary; with involucral bracts (each flower in Phyllangium enclosed by a bilobed, foliaceous involucre), or without involucral bracts. The involucres of Phyllangium accrescent. Flowers bracteate, or ebracteate; ebracteolate (assuming the involucre of Phyllangium is not bracteolar); small; regular; 5 merous (in Mitreola), or 4 merous (in Mitrasacme and segregates); tetracyclic. Floral receptacle with neither androphore nor gynophore. Free hypanthium absent. Hypogynous disk absent.
Perianth with distinct calyx and corolla, or petaline (Phyllangium); 4, or 8, or 10; 2 whorled; isomerous. Calyx 4, or 5; 1 whorled, or 2 whorled (two decussate pairs in Schizacme archeri); gamosepalous (usually), or polysepalous (Schizacme archeri); regular (usually), or unequal but not bilabiate (Schizacme, with one pair of sepals or lobes larger); persistent. Corolla 4 (Mitrasacme and segregates), or 5 (Mitreola); 1 whorled; gamopetalous (hairy inside, or bearded in the throat). Corolla lobes markedly shorter than the tube, or about the same length as the tube. Corolla nearly always valvate, or imbricate (in some Mitreola species and Schizacme archeri); urceolate (Mitreola), or campanulate, or hypocrateriform; regular; white, or yellow, or pink, or purple (or mauve).
Androecium 4 (Mitrasacme), or 5. Androecial members adnate (to the corolla tube); all equal; free of one another; 1 whorled. Androecium exclusively of fertile stamens, or including staminodes. Staminodes when present, 1–3; external to the fertile stamens. Stamens 4, or 5; inserted near the base of the corolla tube to midway down the corolla tube; isomerous with the perianth; oppositisepalous; alternating with the corolla members; filantherous (the filaments long to very short). Anthers dorsifixed to basifixed; dehiscing via longitudinal slits; extrorse to introrse (e.g., all states exhibited in Mitrasacme); appendaged (sometimes, in Mitrasacme and Mitreola), or unappendaged. The anther appendages apical (via a straight or curved prolongation of the connective). Pollen grains aperturate; 3 aperturate; colporate.
Gynoecium 2 carpelled. Carpels reduced in number relative to the perianth. The pistil 2 celled. Gynoecium syncarpous; semicarpous to synovarious (in Schizacme and some Mitrasacme and Phyllangium species), or eu-syncarpous to synstylous (i.e., the two styles separate basally and usually but not always joined above, cf. Apocynaceae); superior (usually), or partly inferior (Mitreola, Phyllangium). Ovary 2 locular. Gynoecium median. Ovary sessile. Gynoecium stylate. Styles 1 (mostly, though basally split), or 2 (sometimes, in Mitrasacme sensu lato); apical, or lateral. Stigmas 1, or 1–2; of Mitreola minima wet type; papillate. Placentation axile, or axile to apical (the placentas peltate). Ovules differentiated; 5–50 per locule (‘several to many’); hemianatropous (Mitrasacme), or anatropous (?); unitegmic; tenuinucellate. Endothelium not differentiated. Hypostase absent. Endosperm formation cellular to nuclear (intermediate, in Mitrasacme). Embryogeny solanad (in Mitrasacme).
Fruit non-fleshy. The fruiting carpel when semicarpous, dehiscent; a follicle. Fruit when syncarpous, i.e. usually, dehiscent (truncate or bilobed above), or a schizocarp (sometimes at least partially, in Mitrasacme and Schizacme). Mericarps when splitting into carpels, 2; when recognisable, comprising follicles (splitting along the interior margins of the carpels). Fruit when not assignable to schizocarpic, a capsule (compressed at right angles to the septum, apically truncate or bilobed — i.e., the carpels then constituting partial mericarps). Capsules denticidal, or septicidal and loculicidal (dehiscing apically along the interior margin (loculicidally between the horns) of the carpels, which may be free or more or less connate). Fruit 20–100 seeded (‘many’). Seeds endospermic; small; wingless (globose or compressed). Embryo well differentiated. Cotyledons 2. Embryo straight.
Physiology, biochemistry. Alkaloids absent (at least, from screened species of Mitrasacme). Iridoids detected; ‘Route I’ type. Verbascosides not detected. Cornoside not detected.
Geography, cytology. Temperate, sub-tropical, and tropical. North and tropical America, tropical and subtropical Asia and Australia (Mitreola; Indomalesia, eastern Asia, Australia and New Zealand (Mitrasacme sensu lato). 2n = 20 (Mitreola. Supposed basic chromosome number of family: 10. Ploidy levels recorded: 2.
Taxonomy. Subclass Dicotyledonae; Tenuinucelli. Dahlgren’s Superorder Gentianiflorae; Gentianales. Cronquist’s Subclass Asteridae; Gentianales. APG 3 core angiosperms; core eudicot; Superorder Asteranae; lamiid; Order Gentianales (as a synonym of Loganiaceae?).
Species about 70. Genera 4; Mitreola, Mitrasacme, Phyllangium, Schizacme.
Leeuwenberg 1980, under Loganiaceae. Mitreola and Mitrasacme (plus the recent segregates from the latter, Schizacme and Phyllangium) are long standing and closely related members of the traditional, manifestly heterogeneous Loganiaceae (cf. their proximity in the Genera Plantarum (1876)). Struwe and Albert (1994) retained them with Logania in their sensu stricto version of Loganiaceae. On the basis of the more detailed descriptions compiled here, however, they are no closer to Logania than to Gentianaceae. The range of gynoecia and fruits is reminiscent of Apocynaceae, which seem distant in other respects. See comments under Loganiaceae.