Woody Stems are the stems of plants who have been growing for two years or more. After two years, a plant's stem thickens to accommodate growth and increased circulation in a plant. Probably the most obvious example of a woody stem is a tree trunk. Trees are a special woody stem and consequently have unique parts that other woody stems do not. For example, a tree contains heartwood, sapwood, and cork. Even so, it is not uncommon for woody stems to start out as a bush or plant and eventually grow into a tree.