| # The Families of Angiosperms |

L. Watson and M.J. Dallwitz

### Pterostemonaceae (Engl.) Small

~ Grossulariaceae sensu lato, Iteaceae.

Habit and leaf form. Much-branched shrubs; resinous. 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 small, caducous. Lamina margins dentate. Domatia occurring in the family; manifested as pockets.

Leaf anatomy. The leaf lamina dorsiventral. 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 without secretory cavities. The mesophyll containing crystals (around the veins, and sometimes in the palisade). The crystals mainly druses.

Axial (stem, wood) anatomy. Secretory cavities absent. Cork cambium present. Nodes tri-lacunar. Secondary thickening developing from a conventional cambial ring. The axial xylem with vessels (many of them ‘fibriform’).

The wood diffuse porous. The 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). The vessels without vestured pits. The axial xylem with tracheids; with fibre tracheids; with libriform fibres. The parenchyma apotracheal (diffuse, sparse). ‘Included’ phloem absent. The wood not storied.

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

Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’; in cymes, or in corymbs. The ultimate inflorescence units 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, phytochemistry. C3. C3 physiology recorded directly in Pterostemon. Anatomy non-C4 type. Flavonols present; quercetin.

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 III core angiosperms; core eudicot; unplaced at Superordinal level. APG IV Order Saxifragales (as a synonym of Iteaceae?).

Species 2, or 3. Genera 1; only genus, Pterostemon.

General remarks. The compiled descriptions show numerous differences from Iteaceae (q.v.) in conventional inflorescence, corolla, androecium, gynoecium and seed characters, as well as in ‘esoteric characters’ (pollen morphology, flavonol data).

Illustrations. • Pterostemon mexicanus: Nat. Pflanzenfam. III (1891). • Pterostemon mexicanus: Hutchinson.

We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, genera included in each family, and classifications (Dahlgren; Dahlgren, Clifford, and Yeo; Cronquist; APG). See also Guidelines for using data taken from Web publications.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 1992 onwards. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 8th June 2026. delta-intkey.com’.

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