heartwood [ HAHRT-wood ] noun: the hard central wood of the trunk of a tree
sapwood [ SAP-wood ] noun: the younger, softer wood of a tree that is between the vascular cambium and the heartwood; usually lighter in color than heartwood and more active in conducting water
xylem [ ZAHY-luhm, -lem ] noun: a complex vascular tissue consisting of fibrous and woody elongated cells that carries water and dissolved minerals from the roots through the stem and leaves; xylem helps support the plant and is the main constituent of wood. [Compare with phloem.]
Xylem is the wood of a woody plant. It has three types of cells: conductive elements and fibers, both of which are dead cells, and living parenchyma cells. In the newer, outer wood layer, called sapwood, are the conductive elements that allow water and minerals to move from the roots to the crown.
As the tree ages and the vascular cambium layer produces new sapwood, the inner xylem cells compact and harden so they are no longer able to transport water. They form the central support column known as heartwood, which is composed mostly of cellulose fibers bound together by lignin. Softwood trees, like willows, poplars, and usually conifers, do not have heartwood (Zuckerman & Davidson 2015). In conifers, the main components of wood —up to 90% of the volume—are conductive elements with only a small portion of xylem parenchyma cells (Słupianek et al. 2021).
Axial parenchyma originates from the vascular cambium and is oriented along the length of the tree in the growth rings. Radial parenchyma also originates from the vascular cambium. It constitutes the wood rays, which are oriented like the spokes of a wheel, perpendicular to the rings and extending to the pith, the center and oldest part of the tree.
References
Anatomy of a Tree. Arbor Day Foundation. (accessed April 14, 12023).
Axial parenchyma. Tree Biology. Landscape Plants. University of Florida. (accessed April 15, 2023).
How a Tree Grows. University of New Hampshire Extension. (accessed April 11, 2023).
Patten AM, Vassão DG, Wolcott MP, Davin LB, Lewis NG. 2010. 3.27 – Trees: A Remarkable Biochemical Bounty.
Editors: Hung-Wen (Ben) Liu, Lew Mander. Comprehensive Natural Products II. Pages 1173-1296. doi: 10.1016/B978-008045382-8.00083-6. ISBN: 9780080453828.
Słupianek A, Dolzblasz A, Sokołowska K. June 18, 2021. Xylem Parenchyma-Role and Relevance in Wood Functioning in Trees. Plants (Basel). 19;10(6): 1247. doi: 10.3390/plants10061247. PMID: 34205276; PMCID: PMC8235782.
Zuckerman LD, Davidson MW. 2015. Glossary of Terms. Tree Collection. Molecular Expressions. Florida State University. (accessed April 15, 2023).