GUAIACUM, a genus of trees of the natural order _Zygophyllaceae_. The guaiacum or lignum-vitae tree (Ger. _Guajakbaum_, _Franzosenbaum_, _Pockenholzbaum_; Fr. _Gayac_, _Gaiac_), _G. officinale_, is a native of the West Indies and the north coast of South America, where it attains a height of 20 to 30 ft. Its branches are numerous, flexuous and knotted; the leaves opposite and pinnate, with caducous (falling early) stipules, and entire, glabrous, obovate or oval leaflets, arranged in 2 or, more rarely, 3 pairs; the flowers are in axillary clusters (cymes), and have 5 oval pubescent sepals, 5 distinct pale-blue petals three times the length of the sepals, 10 stamens, and a 2-celled superior ovary. The fruit is about 3/4 in. long, with a leathery pericarp, and contains in each of its two cells a single seed (see fig.). _G. sanctum_ grows in the Bahamas and Cuba, and at Key West in Florida. It is distinguished from _G. officinale_ by its smaller and narrow leaflets, which are in 4 to 5 pairs, by its shorter and glabrous sepals, and 5-celled and 5-winged fruit. _G. arboreum_, the guaiacum tree of Colombia, is found in the valley of the Magdalena up to altitudes 800 metres (2625 ft.) above sea-level, and reaches considerable dimensions. Its wood is of a yellow colour merging into green, and has an almost pulverulent fracture; the flowers are yellow and conspicuous; and the fruit is dry and 4-winged. The lignum vitae of commerce, so named on account of its high repute as a medicinal agent in past times, when also it was known as _lignum sanctum_ and _lignum Indicum_, _lignum guaycanum_, or simply _guayacan_, is procured from _G. officinale_, and in smaller amount from _G. sanctum_. It is exported in large logs or blocks, generally divested of bark, and presents in transverse section very slightly marked concentric rings of growth, and scarcely any traces of pith; with the aid of a magnifying glass the medullary rays are seen to be equidistant and very numerous. The outer wood, the sapwood or alburnum, is of a pale yellow hue, and devoid of resin; the inner, the heartwood or duramen, which is by far the larger proportion, is of a dark greenish-brown, contains in its pores 26% of resin, and has a specific gravity of 1.333, and therefore sinks in water on which the alburnum floats. Owing to the diagonal and oblique arrangement of the successive layers of its fibres, the wood cannot be split; and on account of its hardness, density and durability it is much valued for the manufacture of ships' pulleys, rulers, skittle-balls, mallets and other articles. [Illustration: From Bentley & Trimen's _Medicinal Plants_, by permission of J. & A. Churchill. Guaiacum or Lignum Vitae, _Guaiacum officinale_ shoot-bearing leaves and flowers. 1, Fruit; 2, Vertical section of fruit, showing the solitary pendulous seed in each chamber. All about 1/2 natural size.]