Arborvitae, Linnaeus  1753
Cupressaceae

Thuja - Arborvitae description

Evergreen shrubs to giant trees with one to several trunks clothed with fibrous, furrowed bark peeling in vertical strips and often densely branched from the base with upwardly arcing branches, the lower branches sometimes rooting to form new trunks. Branchlets in flattened, fernlike sprays, held more or less horizontally, those contributing to the permanent framework of branches (long shoots) only weakly differentiated from the more compact, highly branched ones that are shed as intact sprays after a few years (short, shoots). Virtually all branching arising from the axils of lateral leaves on alternate sides of the spray or just on the forward (away from the trunk) side, with only a tiny fraction of opposite branching. Tips of branchlets enclosed by arrested ordinary foliage leaves, thus without specialized winter buds. Seedling leaves in alternating quartets, needlelike, standing out from and well-spaced on the stem, seedling phase generally short-lived, adult branchlets appearing by the second year. Adult leaves in alternating pairs, scalelike, dense, the bases running down onto and completely clothing the branchlets, with paler (silvery in Thuja koraiensis) stomatal zones beneath, the alternate pairs dissimilar. Lateral pairs keeled, usually not quite touching at their bases, thus separated by the flat successive pairs of facial leaves, which themselves barely overlap, with or without a visible resin pocket on the face.

Plants monoecious. Pollen cones numerous, single at the ends of most short branchlets on those sprays that bear them. Individual cones spherical to slightly oblong, with three to six alternating pairs of pollen scales, each with two to four pollen sacs. Pollen grains small (20-35 µm in diameter), nearly spherical, almost featureless. Seed cones numerous, single at the ends of short branchlets, maturing in a single season and remaining intact as the scales spread. Individual cones oblong, with four or six alternating pairs of thin, woody scales. Each cone scale with the bract almost fully united with the seed scale, completely covering it externally, except for a narrow rim, and ending with a small point below the tip. Middle two or three pairs of scales each usually with two (one to three) seeds. Seeds oval, with two narrow, equal wings running the whole length of the body and a little beyond, the wings derived from the seed coat. Cotyledons two, each with one vein. Chromosome base number x = 11.

Wood pleasantly fragrant, light, soft, and brittle but very decay resistant, with a narrow band of nearly white sapwood sharply contrasting with the light yellowish brown or pale brown to reddish to somewhat coarse, with well-defined growth rings marked by a gradual to abrupt transition to a narrow to middling, dark band of much smaller, thicker-walled latewood tracheids. Resin canals absent but with sparse individual resin parenchyma cells widely scattered or concentrated in open bands in some growth increments.

Stomatal zones densely packed with stomates, covering most of the downward-facing side of all lateral leaves and broad regions on either side of the midline of only the downward-facing member of each facial pair, the upper side of lateral leaves and the upward-facing facial leaves with stomates largely confined to areas hidden by leaf overlap. Each stomate sunken beneath and partially hidden by the four to six surrounding subsidiary cells (or with a partial second ring of another one or two of them) which are topped by a steep but lobed and incomplete Florin ring. Many other epidermal cells in the stomatal zones also bearing knoblike papillae. Midvein single stranded, close to the twig axis in the attached leaf base and soon ending in the free tip, flanked by small wedges of transfusion tissue, unaccompanied by resin canals, but sometimes extending to the same level as the resin pocket (when present). Photosynthetic tissue with a well-developed palisade layer only on those leaves or faces covering the upper side of the branchlets below the epidermis and thin accompanying hypodermis, the remaining cells of the spongy mesophyll sometimes somewhat elongated but even then much looser and more open than the palisade parenchyma.

Five species in North America and eastern Asia. Thuja is an ancient Greek name for a resinous evergreen tree appropriated by Linnaeus for these decidedly non-Greek trees. The genus has been exceptionally productive of cultivars, especially in northern white cedar (Thuja occidentalis), with selection for variations in growth rate, size and shape of mature trees (or bushes), and arrangement of foliage sprays as well as for silver and yellow color variants. The foliage may sometimes be hard to distinguish from that of Chamaecyparis. This is especially true for permanently juvenile horticultural forms, and some such cultivars are not easily assigned to their proper genus.

The seed cones of Thuja are diagnostic and are one of the reasons that Platycladus orientalis is no longer included in this genus. Fossil specimens of Thuja with both cones and foliage have been found in Paleocene sediments (about 60 million years old) of Spitsbergen Island and Ellesmere Island, both in the arctic, in Eocene sediments (about 50 million years old) of British Columbia, and in Miocene sediments (about 20 million years old) of Japan and the Russian Far East. The oldest forms have more cone scales (eight or nine pairs) than the living species. Foliage resembling that of Thuja has an even longer record, extending back to the Cretaceous, but cannot be securely identified with this genus. One cone-bearing shoot from the Cretaceous of Alaska, about 90 million years old, that was recently assigned to the genus appears to be misidentified.

Morphological evidence and DNA studies show that the closest relative of the Thuja arborvitaes is the Hiba arborvitae (Thujopsis) and that these two genera fall at the base of the phylogeny of the northern hemisphere subfamily Cupressoideae, sister to all other genera in the subfamily. This would make a Cretaceous record for Thuja (or its common ancestor with Thujopsis) phylogenetically plausible so, despite the inadequacy of the Alaskan record just mentioned, the genus may yet be confirmed in Cretaceous sediments.

References

• Farjon, A. (2010). A Handbook of the World's Conifers. Koninklijke Brill, Leiden.
• Eckenwalder, J.E. (2009) Conifers of the World: The Complete Reference. Timber Press, Portland.
• IUCN Red List of Threatened Species, International Union for Conservation of Nature and Natural Resources. Cambridge, UK /Gland, Switzerland

Copyright © Aljos Farjon, James E. Eckenwalder, IUCN, Conifers Garden. All rights reserved.