A term for the cells in wood with a long stretched shape. The fibres run almost exclusively lengthwise in the trunk cross section. Exceptions are the wood rays that along with their cells are arranged in a radial direction in the wood. They lie crosswise. The wood matter consists of millions of such cells and can be described in simplified terms as a bundle of tubes which are arranged lengthwise to one another. This results in the anisotropy. This describes the different nature of material properties of wood depending on the grain direction. Wood displays different behaviour along and across the grain, for example, in terms of: stress (compressive forces and traction forces), shrinkage and swelling as well as thermal conductivity.