In old trees the greater part of secondary xylem is dark brown and resistant to insect attack due to : 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) A and B only
Solution
The dark brown, insect-resistant secondary xylem of old trees is heartwood (duramen).
As per NCERT, heartwood is formed due to deposition of organic compounds like tannins, resins, oils, gums, aromatic substances and essential oils in the central or innermost layers of the stem; these secondary metabolites are deposited in the lumen of its vessels ⇒ A and B are correct.
C and D refer to the outer / peripheral layers of the stem ⇒ incorrect (heartwood is central).
E describes sapwood (living parenchyma, functionally active xylem) ⇒ incorrect.
∴ A and B only.
Answer: (1)