This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
Evergreen trees or shrubs with compressed branchlets and imbricated often tuberculate scale-like leaves. Flowers monoecious. Fruit conoid, composed of overlapping scales affixed by the base, and destitute of recurved prickles. Seeds usually 2, with a marginal wing. As thus characterised, this genus is limited to the American species of Arbor Vitse, though the Eastern species, or at least some of the forms, are usually known in gardens under the name of Thuja. The principal difference is in the scales of the fruit, which are shield-like and tubercled in Biota, or the Chinese Arbor Vitae. The generic name is from Ovov, an odoriferous tree used for incense.
1. Th. gigantea, syn. Th. Menziesii, and Th. Lobbii of English gardens. This is a very ornamental fast-growing tree, attaining a gre,at size in the valleys of the Rocky Mountains. Branchlets slender, flexible, and very numerous, compressed, covered with scale-like finely pointed leaves, which are of a very bright shining green on the upper side of the branches and glaucous on the lower side. Leaves destitute of tubercles, persistent and changing to brown on the older branches. Fruit almost exactly like that of the common American Arbor Vitse. This is perfectly hardy and greatly exceeds the following species in elegance and gracefulness of habit.
There is so much confusion in the nomenclature of Conifers and many of them have so many synonyms, that we do not feel quite confident that we have adopted the correct name for the species usually called Thuja Lobbii in gardens. But there is no doubt that the plant bearing the name of Thuja gigantea in many collections is the true Libocedrus decurrens. The only matter for surprise is that these two wholly dissimilar shrubs should have been mistaken the one for the other.
2. Th. occidentalis. American Arbor Vitse. - In the low swampy districts of the Northern States of the Union and in Canada this forms a compact tree from 20 to 50 feet high, but with us it rarely exceeds the dimensions of a large shrub. The branchlets are crowded, compressed, and rather massive, and the small leaves quadrifariously imbricated. The leaves of the lateral ranks are destitute of tubercles, whilst those in the centre on the upper and lower surfaces of the branchlets are mostly furnished with a conspicuous tubercle immediately below the acute apex. The foliage of this is of a bright light green in Summer, but like nearly all of the Arbor Vitaes it changes to a rusty hue in Winter, hence it should be avoided where bright cheerful verdure is desirable at that season of the year. The fruit is small, with about six slightly coriaceous persistent bracts. This is one of those trees whose seeds produce an infinity of more or less distinct individuals, but these differences are so slight as to be unworthy of perpetuation. There are, however, several very distinct varieties or races which come true from seed, whilst others can only be preserved by nonsexual propagation. Amongst the most interesting we may enumerate the variety Sibirica, syn. Wareana, Tartarica, etc. This is commonly called the Siberian Arbor Vitae, though its native country is unknown. In fact, it is said to have been raised from seed in the nursery of a Mr. Weire at Coventry, but whence the seeds came is not stated. It forms a compact, conical, very densely branched bush, with rather smaller closer rather obtuse leaves in which the tubercles are less prominent. Besides the dwarfer, more bushy habit of this variety, we must not omit to mention that the foliage is of a darker green. In addition to the above names it has received about a dozen others, but it usually bears one of the three quoted. Th. o. plicata is another well-known form remarkable for the twisted branchlets being in pairs, giving it a plaited appearance. There is a variety of this variegated with yellow and green. The variety pendula has drooping branches with tufts of branchlets at their extremities; cristata is a similar form; pumila, minima, and compacta are dwarf bushes; ericoides is remarkable on account of most of the leaves being linear and spreading, showing a tendency to develop two kinds of leaves in this genus, a common occurrence in Junipers. The variegated varieties have little to recommend them, though that called Vervameana is rather more distinct than some of them.