* Complexity: Tree bark is made up of a complex mixture of different organic compounds. These include:

* Cellulose: The primary structural component of bark.

* Lignin: A rigid polymer that provides strength and waterproofing.

* Tannins: Polymers that give bark its color and astringent properties.

* Resins: Organic compounds that act as a protective sealant.

* Sugars: Simple carbohydrates.

* Other compounds: Depending on the species of tree, bark can contain a variety of other substances like oils, pigments, and minerals.

* Heterogeneity: The composition of bark can vary greatly even within a single tree, depending on the age of the bark, the location on the trunk, and environmental factors.

Pure substances are defined as having a fixed composition and uniform properties throughout. Since tree bark is a complex mixture of compounds that can vary in composition, it does not meet the definition of a pure substance.