Describe characteristics of heartwood.

Explanation

### Step-by-Step Solution:

1. **Definition of Heartwood**: Heartwood is the central or inner part of a tree's trunk that is dark in color. It is also referred to as duramen.

2. **Formation**: Heartwood is formed during the secondary growth of dicotyledonous stems. This growth involves the accumulation of secondary xylem.

3. **Composition**: The cells that make up heartwood are degenerated, meaning they are no longer functional in terms of water conduction.

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