Crossosomatales

Crossosomatales, rockflower order of dicotyledonous flowering plants, belonging to the basal rosid group of the Angiosperm Phylogeny Group II (APG II) botanical classification system (see angiosperm). The order is a heterogeneous assemblage of eight families, which can be broken down into two groups. The first group consists of the families Crossosomataceae, Stachyuraceae, Staphyleaceae, and Guamatelaceae, and the second includes Aphloiaceae, Geissolomataceae, Ixerbaceae, and Strasburgeriaceae. Most members of the order are woody shrubs or trees of the northern temperate region that contain copious amounts of crystals, with toothed leaf margins. Their flowers typically possess a perianth with four or five sepals and petals, a well-developed nectary, a superior ovary of up to five free or fused carpels, and dehiscent or berrylike fruits with seeds that often have an aril to facilitate animal dispersal.

Crossosomataceae, or the rockflower family, contains four genera of xeromorphic shrubs that grow on rhyolite in the southwestern United States and adjacent region of Mexico. The family is adapted to high temperatures, arid conditions, and nutritionally deficient soils by their development of tough, reduced leaves and C₄ photosynthesis. The plants, known locally as rockflower, apachebush, and greasebush, are not familiar in cultivation or of any great economic importance.

Stachyuraceae is composed of a single genus (Stachyurus) of five species that grow from the Himalayas to Japan. The evergreen or deciduous trees have inflorescences that resemble poplars and aspens, for which reason they were previously placed near the family Salicaceae. Some members of Stachyurus are grown as ornamentals and flower well before the leaves are fully developed.

Most members of Staphyleaceae, or the bladdernut family, are deciduous trees restricted to the northern temperate region, but some species range as far south as Bolivia and Malaysia. Staphylea (bladdernut) consists of 11 species in the temperate region and is often cultivated. Turpinia, with at least 10 species, is native to tropical America and Southeast Asia, where various species are used locally for their timber. Euscaphis consists of the single species E. japonica (Korean sweetheart tree), which is native to China, Korea, and Japan; its fruit is used to prepare herbal medicine.

Guamatelaceae consists of a single genus (Guamatela) and one species (G. tuerckheimii), which is native to Central America. It is unusual for the order in many characteristics, including the absence of any endosperm and a well-developed hypanthium (cup-shaped structure that supports the sepals, petals, and stamens of the flower).

Four other small families of shrubs and trees are sometimes included in Crossosomatales. Each has but a single species. These include Aphloiaceae, found from East Africa to the Seychelles; Geissolomataceae, endemic to the Cape Province of South Africa; and the closely related Ixerbaceae and Strasburgeriaceae, restricted to New Zealand and New Caledonia, respectively.