Question: 99: In old trees the greater part of secondary xylem is dark brown and resistant to insect attack due to :
(a) secretion of secondary metabolities and their deposition in the lumen of vessels.
(b) deposition of organic compounds like tannins and resins in the central layers of stem.
(c) deposition of suberin and aromatic substances in the outer layer of stem.
(d) deposition of tannins, gum, resin and aromatic substances in the peripheral layers of stem.
(e) presence of parenchyma cells, functionally active xylem elements and essential oils.
Choose the correct answer from the options given below:
(1) (d) and (e) Only
(2) (b) and (d) Only
(3) (a) and (b) Only
(4) (c) and (d) Only
Answer: Option (3)
Explanation:
In old trees, the inner region of secondary xylem forms heartwood,
which appears dark brown in colour.
Heartwood becomes resistant to insect attack due to the secretion of secondary metabolites
that are deposited in the lumen of xylem vessels.
These secondary metabolites include organic compounds such as tannins and resins.
The deposition of tannins and resins mainly occurs in the central layers of the stem,
which constitute the heartwood.
The outer layers of the stem form sapwood, which is lighter in colour and functionally active in water conduction.
Suberin deposition is associated with cork tissue, not with secondary xylem.
Therefore, statements (a) and (b) correctly explain the dark colour and
insect resistance of secondary xylem in old trees.
Hence, the correct answer is option (3).