Question

In old trees the greater part of secondary xylem is dark brown and resistant to insect attack due to:

1. Secretion of secondary metabolites and their deposition in the lumen of vessels.
2. Deposition of organic compounds like tannins and resins in the central layers of the stem.
3. Deposition of suberin and aromatic substances in the outer layer of the stem.
4. Deposition of tannins, gum, resin and aromatic substances in the peripheral layers of the stem.
5. Presence of parenchyma cells, functionally active xylem elements and essential oils.

Choose the correct answer from the options given below:

• (a) and (b) only
• (c) and (d) only
• (d) and (e) only
• (b) and (d) only

Answer 1

The Correct Option is A

In the context of old trees, the majority of the secondary xylem becomes dark brown and is known to be resistant to insect attack. This is primarily due to specific processes that occur within the tree:

Explanation:

1. The secretion of secondary metabolites and their subsequent deposition in the lumen of vessels makes the xylem tough and less palatable to insects. These metabolites include various compounds that can deter insect predation.

2. The deposition of organic compounds like tannins and resins within the central layers further enhances the tree's defense mechanism. Tannins are well-known for their anti-herbivory properties, and resins can provide a physical barrier to invaders.

Thus, the processes described in (a) and (b) effectively contribute to the transformation of the secondary xylem in old trees, providing both durability and resistance to biotic stress such as insect attacks.

Conclusion: Based on the explanations above, the correct options that describe why the secondary xylem in old trees is dark brown and resistant to insect attack are option (a) and (b) only.