The wood splits very irregularly, owing to the oblique and varying course taken by the wood fibres. Both these and the vessels - in fact, all the elements of the heartwood - are filled with a dark resin, which is sometimes also found in cavities in the trunk. It exhales, when warmed, a faint aromatic odour, recalling benzoin, and has, when chewed, an acrid taste. Its toughness and hardness render it valuable for many technical purposes, it being used in making blocks, pulleys, etc. The chips or turnings, in which state it is usually employed in pharmacy, should consist of the dark-coloured heart-wood alone, but they are frequently mixed with the pale yellow sapwood from which they may be separated by a 30 per cent. solution of sodium chloride in which the splinters of heartwood sink. The sapwood contains about 3 per cent. of resin, not identical with the resin of the heartwood.

The student should observe

(a) The pale colour of the sapwood,

(b) The dark greenish brown heartwood,

(c) The distribution of the vessels.

Microscopical Characters

The vessels are large and isolated, often extending from one medullary ray to the next. The medullary rays are one cell wide and three to six cells high. The wood-fibres are abundant and have very thick walls. The wood parenchyma occurs in narrow bands and some of the cells contain prismatic crystals of calcium oxalate.

Constituents

The heartwood of guaiacum contains between 20 and 25 per cent. of resin, which has been found to consist of a- and β-guaiaconic acids, guaiaretic acid, and guaiacic acid. Guaiaconic acid is converted by oxidising agents into guaiac blue and accordingly tincture of guaiacum wood strikes a deep blue colour with dilute solution of ferric chloride, a reaction which is useful in identifying the wood. (Compare 'Guaiacum Resin.')

Guaiacum wood also contains guaiacsaponic acid and guaiac saponin, two non-toxic bodies belonging to the class of saponins; they are present in larger quantity in the sapwood than in the heart-wood. Guaiaguttin, which resembles gutta-percha, is also present.

Guaiaconic acid, though a characteristic constituent of the wood, has been found in other woods (e.g. species of Bulnesia and Porlieria), and its presence therefore is not an infallible diagnostic character of guaiacum wood.

Uses

Guaiacum has a local stimulant action which is sometimes useful in sore throat. The resin is used in chronic gout and rheumatism, whilst the wood is an ingredient in the compound concentrated solution of sarsaparilla, which is used as an alterative in syphilis.

Substitutes, Etc

Commercial guaiacum wood turnings frequently contain the sap wood as well as chips of other woods. These may be detected by their floating in brine and by the amount of alcoholic extract which should not be less than 22 per cent.