Wood is a heterogeneous mixture. There are differences in the grain that show the material is not uniformly mixed. This allows certain areas of the sample of wood to have different properties than other areas.
In a homogeneous mixture, an analysis of any part of the whole produces the same results as another part of the mixture. A 5-milliliter sample from a 1-liter salt solution that is homogeneous in nature would reveal the same concentration if the chemist sampled from the bottom of the beaker or the top. Doctors use the fact that one's blood is a homogeneous mixture in diagnostic testing.