TROCHODENDRALES

order of dicotyledonous flowering plants comprising two families (Trochodendraceae and Tetracentraceae), each represented by a single small- to medium-sized tree species. The order is noteworthy as an evolutionary primitive group isolated from other living flowering plants: its wood, for example, is without the specialized water-conducting cells (vessel cells) found in nearly all other flowering plants. In their place is a more primitive cell type, the tracheid, common in gymnosperms and some other nonflowering plants; the tracheid is a long, spindle-shaped cell with numerous lateral openings, or pits, instead of openings at the ends. These cells are arranged in overlapping layers. Despite the primitive wood, both species have flowers that are considered highly specialized and somewhat advanced, evolutionarily. The wheel tree (Trochodendron aralioides), of South Korea, Japan, and Taiwan, is a small broadleaf evergreen tree up to 12 m (about 40 feet) in height with pinnate leaves (i.e., the leaves have a midrib from which lateral veins arise, comblike) and flowers in clusters at the branch tips. The flowers lack sepals and petals (the outer and inner floral whorls or showy flower parts) but have an array of about 70 stamens (male pollen-producing structures) surrounding 5 to 11 or more partially fused carpels (female ovule-bearing structures); the flowers are thus bisexual. It is from the ring of stamens, which has the appearance of a wheel, that the genus receives its name, which means roughly, wheel tree. Trochodendron flowers are insect-pollinated. The plant is sometimes cultivated as an ornamental. Tetracentron sinense, of central and south-central China and northern Myanmar (Burma), is a medium-sized tree, 4.5 to 27 m tall, with palmately (fingerlike) veined leaves and small, wind-pollinated flowers arranged in dangling, slender catkins. It is also cultivated occasionally as an ornamental.