ABSTRACT
Many timbers develop gums and resins in the heartwood while the moisture content of the heartwood of most timbers is appreciably lower than that of the sapwood in the freshly felled state. However, in exceptional cases, high moisture contents can occur in certain parts of the heartwood. Known as wetwood, these zones are frequently of a darker colour than the remainder of the heartwood and are thought to be due to the presence of microorganisms, which produce aliphatic acids and gases. Radial growth of the trunk must accommodate the existing branches and this is achieved by the structure that is known as the knot. The distribution of cell types and their sizes is also used as a means of timber identification. Interconnection by means of pits occurs between cells to permit the passage of mineral solutions and food in both longitudinal and horizontal planes. Simple pits, bordered pit and semi-bordered pits are the basic types of pit.
