Thujopsis, which were united with it by Bentham and Hooker. So limited, it comprises 5 species, and is divided into the two following sections:—
I. Euthuya. Cones with thin, coriaceous mucronulate scales, those of the 2 or 3 middle ranks being fertile. Seed thin, with lateral wings and a minute hilum. This section comprises 4 species. Thuya occidentalis and Thuya plicata of North America, Thuya sutchuenensis of central China, and Thuya japonica of Japan.
II. Biota. Cones with thickened, conspicuously umbonate scales, which are fleshy when young, almost ligneous when ripe; those of the lowest two ranks fertile. Seed thick, without wings, the hilum being large and oblong. This section includes one species, Thuya orientalis of north China.
The Thuyas resemble considerably in foliage and habit the flat-leaved cypresses. The latter are best distinguished by their fruit, which consists of peltate scales fitting closely by their edges. In a subsequent part, the peculiarities, as regards the branch systems and leaves, of these cypresses (Cupressus Lawsoniana, nootkatensis, thyoides, obtusa, and pisifera) will be described, and may then be compared with those now given below for the four species of Thuya in cultivation.
In the discrimination of the Thuyas, in addition to the characters shown by the bark, mode of branching, and fruit, the primary and secondary axes of the branchsystems give good marks of distinction. These axes are markedly flattened in Thuya occidentalis, terete in the other species. In Thuya orientalis the branch-systems stand in vertical planes, the inner edges of which are directed towards the stem of the tree. In ordinary forms of the other three species they are arranged in horizontal planes. The leaves on the main axes in each species differ as follows: —
1. Thuya plicata:[1] widely spaced, long, ending in long, fine, free points, which are parallel to the axis; glands inconspicuous or absent. Under surface of the foliage usually marked with white streaks.
2. Thuya japonica: placed closely together, shoots ending in short, rigid, thick, triangular points, directed outwards at an acute angle; glands absent. Under surface of the foliage conspicuously marked with broad white streaks.
3. Thuya occidentalis: widely spaced, ending in long, fine points, which are parallel to the axis; glands raised, large and conspicuous on the flat leaves. Under surface of the foliage pale green; white streaks inconspicuous or absent.
4. Thuya orientalis: widely spaced, ending in short triangular free points, which are not rigid, and are directed slightly outwards at an acute angle: flat leaves marked by longitudinal glandular depressions. Under surface of the foliage pale green, without white streaks.
Thuya sutchuenensis, Franchet,[2] is a small tree occurring in north-east Szechuan in central China, where it was discovered by Pere Farges growing at an altitude of 1400 feet. The branchlets are much flattened, thin in texture, and practically glandless. Cones composed of 8 obovate scales, the apices of which are slightly thickened. This species has not been introduced into cultivation.
