In old trees, central dark coloured, non-conducting part of secondary xylem is referred to as

Options

A. heartwood
B. sapwood
C. softwood
D. hardwood

Explanation

### Step-by-Step Solution

1. **Understanding the Structure of Xylem**: In old trees, the xylem is divided into primary and secondary xylem. The primary xylem is formed during the initial stages of growth, while the secondary xylem is produced later through the activity of the cambium.

2. **Role of Cambium**: The cambium is a layer of actively dividing cells located between the xylem and phloem. It contributes to the growth of the tree by producing new xylem cells towards the inside and new phloem cells towards the outside.

3. **Formation of Secondary Xylem**: As the cambium continues to divide, it generates secondary xylem towards the pith (inner side) of the tree. Over time, this secondary xylem accumulates and forms a significant part of the tree's structure.

4. **Darkening of the Central Part**: With age, the central part of the secondary xylem accumulates organic materials such as resins, tannins, and oils. These materials lead to a darkening of this region.

5. **Characteristics of the Central Part**: The central dark-colored part becomes non-conducting and consists of dead cells with highly lignified walls due to the deposition of these organic compounds.

6. **Terminology**: This central dark-colored, non-conducting part of the secondary xylem is known as **heartwood**. It is surrounded by the sapwood, which is the outer layer of secondary xylem that is still active in water conduction.

### Final Answer
In old trees, the central dark-colored, non-conducting part of the secondary xylem is referred to as **heartwood**.

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