noun, plural: secondary xylems
The xylem formed as a result of the secondary growth from the vascular cambium
Xylem is the vascular tissue responsible for the conduction of water and nutrients from the roots to the shoots and leaves, especially of terrestrial plants. Based on the stage and origin of growth, a xylem may be classified as primary or secondary.
Secondary xylem is the type of xylem formed from secondary growth. In comparison, the primary xylem forms during primary growth. Because of this, the secondary xylem is associated with lateral growth rather than vertical growth as in the primary xylem. Another difference lies on the type of cambium that gives rise to them. The primary xylem comes from the procambium whereas the secondary xylem grows from the vascular cambium.
Secondary xylem is absent in non-woody plants but is present in trees and shrubs. Its cell walls are thickened by deposition of lignin, thereby, rendering mechanical support to such plants. Secondary xylem consists of tracheids and vessels that are shorter and wider than those of primary xylem. It is also richer in xylem fibers than in primary xylem.
Secondary xylem may show growth rings (or annual rings). In large woody plants, the secondary xylem is differentiated into sapwood and heartwood.
Word origin: Greek xúlon (“wood”)