Question

Youngest layer of secondary xylem is located

• A. In center of stem
• B. Just outside pith
• C. Just outside vascular cambium
• D. Just inside vascular cambium

Answer 1

The correct option is D Just inside vascular cambium

The formation of the vascular cambium is initiated when cells between the columns of vascular tissue connect the cambia inside the columns of vascular tissue to form a complete cylinder around the stem. The cells formed toward the inside are called as secondary xylem or wood and those formed toward the outside of the cambium are called as secondary phloem.
Xylem formation begins when the actively dividing cells of growing root and shoot tips give rise to primary xylem. In woody plants, secondary xylem constitutes the major part of a mature stem or root and is formed as the plant expands in girth and builds a ring of new xylem around the original primary xylem tissues.
When this happens, the primary xylem cells die and lose their conducting function, forming a hard skeleton that serves only to support the plant. Thus, in the trunk and older branches of a large tree, only the outer secondary xylem (sapwood) serves in water conduction, while the inner part (heartwood) is composed of dead but structurally strong primary xylem. So, youngest layer of secondary xylem is located just inside vascular cambium and not in center of stem, outside pith or outside vascular cambium.
Therefore, the correct answer is option D.