Under this heading are grouped together a number of drugs consisting of the wood of the trunk, branch, or root of trees. Medicinal woods are derived from dicotyledonous plants, and the following structural details which may be of assistance in identifying woods apply to those of dicotjdedonous origin.

1. Medullary Rays

Medullary Rays. These are best examined on a smooth transverse and tangential section. On a transverse section they appear as fine, continuous lines radiating from the centre to the periphery. They vary in width in different woods and also frequently in the same wood; the wide ones are usually visible to the naked eye, but they are best examined under the lens. The distance between the rays also varies with the different woods, a variation which may be indicated by stating the number of rays that occur in 5 mm. of the wood.

2. Vessels

Vessels. The size and distribution of the vessels also afford valuable information. The vessels in the spring wood are usually larger and more numerous than those in the autumn wood, and hence more or less well-defined concentric lines are produced; in some woods, however, the distribution is irregularly radial. They are often large enough to be visible to the naked eye (over 0.1 mm.), frequently, however, scarcely perceptible even under the lens (under 0.02 mm.); they may occur singly or be arranged in radial or tangential groups of two or more.

3. Parenchyma

Parenchyma. The parenchymatous tissue of the wood may also exhibit a characteristic distribution usually assuming the form of more or less distinct concentric circles (false annual rings).

The woods dealt with in the succeeding section are easily distinguished from one another by their colour, odour, or taste, but the student should examine them carefully with the lens, as above described, in order to distinguish them from other similar woods that might be substituted for them.