Older parts of plant roots do not normally absorb water because

Analyze the question: The question asks why the older parts of a root don't normally absorb water. It is about where water uptake happens in a root.

Concept: Most water absorption in plants happens through root hairs. These are tiny, thin-walled extensions of epidermal cells found in the piliferous (root hair) zone, just behind the root tip. They greatly increase the surface area for absorbing water and mineral salts from the soil by osmosis and active transport.

As a root grows, the root hairs near the tip are continually formed. Older root hairs die and wither away, and the older epidermis becomes thicker and often suberised (corky), so it is less permeable to water.

Evaluating the options:

• A. They lack xylem: Wrong. Older roots have well-developed xylem, which conducts water upward.
• B. They have small surface area: Wrong. The real issue is that they lack the specialised absorbing structures, not simply that they are small. Older roots are often thicker.
• C. The phloem is dead: Wrong. Phloem transports food, not water, and it is not dead in mature roots.
• D. They lack root hairs: Correct. Older root parts have lost their root hairs, so they cannot absorb water effectively.

**Answer: D.

Exam tip: Remember the phrase "Root hairs = water uptake." If a question asks about absorption by roots, think root hairs. Xylem carries water, phloem carries food, and root hairs absorb water.