Trochodendraceae is the only
family of
flowering plants in the order Trochodendrales. It comprises two extant
genera, each with a single
species[1] along with up to five additional extinct genera and a number of extinct species.[2] The living species are native to south east Asia. The two living species (Tetracentron sinense and Trochodendron aralioides) both have secondary xylem without
vessel elements, which is quite rare in angiosperms. As the vessel-free wood suggests primitiveness, these two species have attracted much taxonomic attention.
Tetracentron and Trochodendron are deciduous or evergreen
trees, which grow to between 20–30 m (66–98 ft) tall, with Trochodendron sometimes sporting umbrella-shaped branches.
Leaves in spirals at the end of the branches (umbrella-like appearance, Trochodendron) or separate (Tetracentron), simple, serrulate or crenulate, with clorantoid teeth, palmately or pinnately divided, brochidodromous or actinodromous, ovate or
obovate, with a cordate to cuneate base and acuminate apex, stalked, with thin
stipules fused with the petiole (Tetracentron) or absent (Trochodendron). Idioblasts present, large, branched, sclerenchymatous in Trochodendron and secretory in Tetracentron.
Stomata laterocytic or cyclocytic, hypostomatic.
Stems without xylematic vessels, with tracheids, heterogeneous
xylem, uni- and multi-seriate, branches clearly differentiated in unifoliate brachyblasts and macroblasts with distichous
phyllotaxis (Tetracentron), with
nodes (1-)3(-multi)-lacunar, with (1)3(-7) leaf stems.
Terminal
Inflorescence in erect, aggregated racemiforms (botryoid or small panicles) (Trochodendron) or defined, axillary, multi-floral amentoid spikes with the flower in whorls of 4 (Tetracentron).
Bracts and bracteoles present or absent.
Perfect
flowers, actinomorphic or dissymmetric, yellowish. Short, sub-conical, or hollow
receptacle. Hypogynous disk absent. Reduced, very thin
perianth, of 4
tepals in 2 decussate whorls (Tetracentron), or at most in a recognizable preantheric state (Trochodendron).
Androecium of 4 decussate
stamens in pairs of 40-70 in a spiral, non-versatile, basifixed, tetrasporangial, latrorso, apiculate
anthers,
dehiscence along 2 longitudinal valves in the theca.
Gynoecium superior (Tetracentron) to slightly semi-inferior (Trochodendron), of 4-11(-17)
carpels, syncarpous (alternating with the stamens in Tetracentron) to semicarpous, the dorsal part of the ovary expanded horizontally in the anthesis, abaxially nectariferous, with sunken stomata, free
styles (stylodious), dry, papillose, decurrent ventral
stigmas, 5-30 anatropous, apotropous, bitegmicous, crassinucelate, pendulous
ovules per carpel,
placentation marginal in 2 series or apicoaxial.
Fruit in ventricidal or slightly loculicidal
capsule or an aggregate of dorsally and ventrally dehiscent semicarpical
follicles, with basal and external styles.
Small, flattened, tapered
seeds, 3–4 mm in length, with lateral, apical, chalazal wings, with thin
testa, with abundant, oily and proteinaceous
endosperm, small
embryo, with 2 cotyledons.
Pollen in small, granular, spheroidal, tricolpate, tectated-columelliform monads (10-20 μm in diameter), the surface with interwoven bars parallel to the edges of the colpus, which are granular.
Pollination is probably myriophyllous, even though Tetracentron shows a clear anemophilous syndrome. The pulverulent seeds are dispersed by the wind (
anemochory). Trochodendron is present in both protandrous and protogynous forms that are self-compatible.
The plants are found in wooded formations, Trochodendron between 300 m and 2.700 m above sea level and Tetracentron between 1.100 m and 3.600 m above sea level.
The fruit and wood genus Nordenskioeldia, along with the associated leaf morphogenus Zizyphoides have been considered part of Trochodendraceae, though phylogenetic analysis by Manchester et al (2020) suggested they might be better placed as a basal stem lineage in the Trochodendrales, rather than as a crown group member of the family Trochodendraceae.[2]
Systematic position
The Trocodendraceae are a group of flowering plants that include the clade
Eudicotyledoneae. In previous systems they have been related with the
Cercidiphyllaceae and the
Eupteleaceae, with which they share some characteristics that can be considered symplesiomorphic or convergent and that have been excluded from the order Trochodendrales because of molecular data leaving the Trocodendraceae isolated. Based on molecular and morphological data, the APW (Angiosperm Phylogeny Website) considers that it constitutes the only family in the Order Trochodendrales (cf.
AP-website).
Classification
The current classification of Trochodendraceae is the
APG IV system published in 2016, which maintains the circumscription of Trochodendraceae used in the
APG III system published in October 2009. Unlike the
APG and
APG II systems, the later systems place the family as the only family in the order Trochodendrales. They also includes Tetracentron, synonymizing Tetracentraceae fully with Trochodenraceae.[4][5]
The
APG II system, of 2003 retained the classification used in the 1998
APG system recognizing Trochodendraceae as a family. APG and APG II did not place the family in an
order, leaving it among the basal lineages of the
eudicots. Both APG systems accepts this as a family of two modern species, but it does allow the option of separating out the family
Tetracentraceae.
The
Cronquist system, of 1981, accepted both Trochodendraceae and Tetracentraceae as families and placed these in the order Trochodendrales, in subclass
Hamamelidae, in class
Magnoliopsida.
Taxa included
The family includes two living genera with very different morphological characteristics:
Palmate leaves, with stipules, deciduous. Perianth of 4 tepals. Stamens 4. Carpels 4. Ovules 5-6 per carpel. Axillary inflorescence in amentoid spike.
TetracentronOliv., 1889. North-east India, Nepal, Bhutan, Burma, western and central China, Vietnam.
Pinnate leaves, without stipules, evergreen. Perianth absent. Stamens 40-70. Carpels (4-)6-11(-17). Ovules 15-30 per carpel. Terminal racemiform inflorescence, erect.
Endress, P.K. (1993). "Trochodendraceae.". Kubitzki, K., Rohwer, J.G. & Bittrich, V. (Editores). The Families and Genera of Vascular Plants. II. Flowering Plants - Dicotyledons. Springer-Verlag: Berlín.
ISBN978-3-540-55509-4.