In old trees, the greater part of secondary xylem is dark brown due to the
a)	Deposition of inorganic material	b)	Deposition of organic material
c)	Activity of cambium	d)	Activity of secondary xylem

(b)

In old trees, the greater part of secondary xylem is dark brown due to the deposition of organic compounds like tanins, resins, oils, gums, aromatic substances and essential oils in the central or innermost layers of the stem. These substances make it hard, durable and resistant to the attacks of microorganisms and insects. The region comprises dead elements with highly lignified walls and is called heart wood