Family Pterostemonaceae

Pterostemonaceae (Engl.) Small

~ Grossulariaceae sensu lato, Iteaceae

Habit and leaf form. Much-branched shrubs; resinous. Plants non-succulent. Leaves alternate (shining glutinous-resinous above, pubescent below); petiolate; non-sheathing; simple; epulvinate. Lamina entire; obovate to orbicular; pinnately veined; cross-venulate. Leaves stipulate. Stipules caducous (small). Lamina margins dentate. Domatia occurring in the family; manifested as pockets.

Leaf anatomy. Hydathodes present (marginal). Mucilaginous epidermis absent. Stomata present; mainly confined to one surface, or mainly confined to one surface and on both surfaces (mainly abaxial); anomocytic. Hairs present; eglandular and glandular; unicellular and multicellular. Complex hairs present; peltate and clavate (or ‘conical’).

Adaxial hypodermis absent. Lamina dorsiventral; without secretory cavities. The mesophyll containing calcium oxalate crystals (around the veins, and sometimes in the palisade). The mesophyll crystals mainly druses.

Stem anatomy. Secretory cavities absent. Cork cambium present. Nodes tri-lacunar. Secondary thickening developing from a conventional cambial ring. ‘Included’ phloem absent. Xylem with tracheids; with fibre tracheids; with libriform fibres; with vessels (many of them ‘fibriform’). Vessel end-walls oblique (mostly), or horizontal (in the widest elements with simple perforations); scalariform (in the narrow elements), or simple (in the wide ones). Vessels without vestured pits. Wood diffuse porous; not storied; parenchyma apotracheal (diffuse, sparse).

Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite.

Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’; in cymes, or in corymbs. The ultimate inflorescence unit cymose. Inflorescences terminal; few-flowered, corymbose cymes. Flowers regular; 5 merous; cyclic; pentacyclic. Free hypanthium seemingly absent (or very slight?).

Perianth with distinct calyx and corolla; 10; 2 whorled; isomerous. Calyx 5; 1 whorled; polysepalous; regular; persistent (erect); valvate. Corolla 5; 1 whorled; polypetalous; imbricate; white; persistent (conspicuous, pubescent, becoming reflexed).

Androecium 10. Androecial members free of the perianth; markedly unequal; free of one another; 2 whorled. Androecium including staminodes. Staminodes 5; internal to the fertile stamens (constituting the inner whorl); non-petaloid (consisting of narrow filaments, toothed above, without anthers). Stamens 5 (representing the outer whorl); isomerous with the perianth; oppositisepalous; alternating with the corolla members; erect in bud; filantherous (the filaments broad, toothed near the apex). Anthers dorsifixed to basifixed; versatile; dehiscing via longitudinal slits; introrse (ovoid). Pollen grains aperturate; 2 aperturate; colporate.

Gynoecium 5 carpelled. Carpels isomerous with the perianth. The pistil 5 celled. Gynoecium syncarpous; synstylovarious; inferior. Ovary 5 locular. Epigynous disk absent. Gynoecium stylate. Styles 1; apical. Stigmas 5 (radiate, separating more with age). Placentation axile. Ovules 4–6 per locule; ascending; embryology not recorded.

Fruit non-fleshy; dehiscent (by contrast with perigynous Rosaceae); a capsule (woody, crowned by the erect sepals and reflexed petals). Capsules septicidal. Fruit few seeded. Seeds non-endospermic; attenuate at either end.

Physiology, biochemistry. Flavonols present; quercetin. C3. C3 physiology recorded directly in Pterostemon. Anatomy non-C4 type.

Geography, cytology. Neotropical. Sub-tropical to tropical. Mexico.

Taxonomy. Subclass Dicotyledonae; Crassinucelli, or Tenuinucelli (?). Dahlgren’s Superorder Corniflorae (?); Cornales (? - cf. Philadelphaceae?). Cronquist’s Subclass Rosidae; Rosales. APG 3 core angiosperms; core eudicot; unplaced at Superordinal level; Order Saxifragales (as a synonym of Iteaceae?).

Species 2. Genera 1; only genus, Pterostemon.