Heart wood is softer than the sap wood and not durable.

Explanation

### Step-by-Step Text Solution:

1. **Understanding Heartwood and Sapwood**:
- Heartwood is the central part of a tree, often darker in color, and is composed of non-living cells. It is also known as central wood.
- Sapwood, on the other hand, is the outer layer of wood surrounding the heartwood and is composed of living cells.

2. **Composition of Heartwood**:
- Heartwood consists mainly of lignified cells. Lignin is a complex organic polymer that provides rigidity and strength to the wood structure.
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