Thujopsis
Hiba arborvitae, P. Siebold & Zuccarini 1844
Cupressaceae
Thujopsis - Hiba arborvitae description
Evergreen trees, often bushy when young. Branches almost horizontal or drooping with age from one or more trunks clothed with fibrous, furrowed bark. Branchlets strongly flattened into fernlike sprays with distinct upper and lower surfaces. Without definite winter buds. Leaves in alternating pairs, needlelike in the short-lived seedling and juvenile phase, which gives way during the second and third years to adult branchlet sprays. Adult leaves scalelike, densely clothing the twigs, with a small resin pocket near the tip, dark green with white stomatal patches above and with conspicuous large waxy white stomatal zones beneath. Leaves of branchlets differentiated into lateral and facial pairs. Leaves of each lateral pair very broad and hooked at the tip, completely separated by the facial pair and seemingly inserted at the same level as them, with a thickened green rim surrounding the stomatal zone beneath. The underside member of the facial pair with a thickened green rim running between the lateral leaves at its tip and also with a thickened green keel.
Plants monoecious. Pollen and seed cones relatively few, single at the ends of short lateral shoots, cupped closely around the base by three or four pairs of green bracts transitional to the foliage leaves of the supporting shoot. Pollen cones cylindrical with six to eight alternating pairs of pollen scales, each scale with three or four pollen sacs. Pollen grains small (25-35 µm in diameter), spherical, largely featureless. Seed cones maturing in a single season and then gaping to release the seeds, almost spherical, with four (or five) alternating pairs of seed scales. Cone scales woody, with a large pyramidal knob occupying most of the face and with little outward sign of the boundary between the united outer bract and inner seed scale portions. Lower two pairs of scales with five seeds each, the upper pairs with one to three seeds. Seeds ellipsoid, narrowly two-winged, the wings arising as outgrowths of the seed coat. Cotyledons two, each with one vein. Chromosome base number x = 11.
Wood fragrant, light, soft, and brittle but decay resistant, the whitish sapwood not strongly differentiated from the yellowish to light brown heartwood. Grain fine and even with occasional coarse streaks, the obvious growth rings marked by a (usually) gradual transition to a fairly narrow band of smaller but not particularly thicker walled latewood tracheids. Resin canals absent but with numerous individual resin parenchyma cells scattered throughout the growth increment or somewhat concentrated in open bands.
Stomates sunken beneath and about half hidden by the five to seven surrounding subsidiary cells, which are often shared by adjacent stomates. Rim of stomate topped by a complete, lobed Florin ring. Subsidiary cells and other epidermal cells of the stomatal zone also often carrying additional knoblike papillae of raised cuticle. Midvein single-stranded, thin, petering out in the free tip of the leaf, flanked by wedges of transfusion tissue, and often accompanied on its outer side in the attached base by a large resin canal that stops before the embedded resin pocket of the leaf tip. Photosynthetic tissue with a (sometimes partial and ill-defined) palisade layer beneath the epidermis and variably developed hypodermis only on the upper side of the branchlets, the remaining space down to the lower side of the branchlets, including the whole lower facial leaf occupied by very loose spongy mesophyll with a scattering of sclereids.
One species in Japan. While fairly hardy, Hiba arborvitae (Thujopsis dolabrata), has a high moisture requirement and is successfully cultivated primarily in areas with a humid climate. Unlike many other ornamental Japanese plants, there has been little cultivar selection in its homeland or elsewhere, with just a handful of cultivars displaying variegated foliage or variations in habit and form from dwarf buns to narrower, faster growing columns.
The close relationship between Thujopsis and typical arborvitaes (Thuja) has been recognized since the sole extant species was first botanically described in 1782 as a species of the latter genus, and this relationship is reflected in the generic name (Greek for “looks like Thuja”), not proposed until 60 years later, in 1842. Halfway in between, in 1817, Richard Salisbury was the first to propose a separate generic name for this species, but his sketchy description prevented taxonomists of the time from recognizing what he was talking about, and his name (Dolophyllum) was never taken up in the literature. Salisbury’s name was finally officially rejected in favor of Thujopsis at the Sixteenth International Botanical Congress in Saint Louis in 1999. Although no DNA studies have been specifically designed to assess the relationship between Thujopsis and Thuja, many broader studies of Cupressaceae included species of these two genera. All studies agree that they are each other’s closest living relatives, while most also show that, together, these two genera are the sister group to all the other northern hemisphere members of subfamily Cupressoideae. They are not, in any event, especially closely related to the superficially similar northern incense cedars (Calocedrus) or to Oriental arborvitae (Platycladus orientalis), which itself is even today still often included as a species of Thuja, especially in horticultural references. The Hiba arborvitae was even briefly included in Platycladus in the mid-19th century, at about the time that it was independently placed in the new separate genus Thujopsis.
The known fossil record for Thujopsis is limited. Many specimens from Tertiary sediments in Europe and a few from North America were assigned to the genus in the 19th century and the first half of the 20th, but none of these reports seems correct. Instead, the only reliable paleobotanical records for the genus are from Japan, to which the genus is restricted (endemic) today. The oldest of these date to the Miocene, more than 10 million years ago, in Hokkaido, north of the present natural distributional limits of the genus.
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.