Trochodendraceae

Trochodendraceae is the only family of flowering plants in the order Trochodendrales. It comprises two extant genera, each with a single species[1] found in 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.

Trochodendraceae
Trochodendron aralioides
Scientific classification
Kingdom: Plantae
Clade: Tracheophytes
Clade: Angiosperms
Clade: Eudicots
Order: Trochodendrales
Takhtajan ex Cronquist
Family: Trochodendraceae
Eichler
Genera

Description
  • Deciduous or evergreen trees, up to 20–30 m, sometimes with umbrella-shaped branches (Trochodendron). Covering hair, scales etc. absent.
  • 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.
  • Hermaphroditic or androdioecious plants.
  • 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.
  • Chromosomal number: 2n = 48 in Tetracentron and 2n = 38, 40 in Trochodendron.

Ecology

Pollination is probably myiophyllous, 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.

Phytochemistry

Flavonoids (quercetin and kaempferol) and proanthocyanidins (cyanidin and delphinidin) are present. Epicuticular waxes are basically composed of nonacosan-10-ol. Tetracentron contains chalcones or dihydrochalcones. Trochodendron contains myricetin. Ellagic acid is absent.

Fossils

Trochodendron and the fossil genus Nordenskioldia (to which the leaf fossils called Zizyphoides are also attributed) was present in the Lower Middle Eocene (49-50 Ma), it has been found in the Republic formation of Washington State (USA), demonstrating that the family had a wider distribution than currently.

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.[2][3]

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.

This segregation would lead to two families with one species each: Tetracentraceae with Tetracentron sinense and Trochodendraceae with Trochodendron aralioides.

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
Theoretical introduction to Taxonomy

The family includes two 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.
Tetracentron Oliv., 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.
Trochodendron Siebold & Zucc., 1839. Japan, Taiwan, Korea.
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References
  1. Christenhusz, M. J. M. & Byng, J. W. (2016). "The number of known plants species in the world and its annual increase". Phytotaxa. 261 (3): 201–217. doi:10.11646/phytotaxa.261.3.1.
  2. Angiosperm Phylogeny Group (2009). An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG III. Botanical Journal of the Linnean Society 161: 105-121.
  3. Angiosperm Phylogeny Group (2016). "An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG IV". Botanical Journal of the Linnean Society. 181 (1): 1–20. doi:10.1111/boj.12385. ISSN 0024-4074.CS1 maint: uses authors parameter (link)
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