Wood is a common name for secondary xylem.
Wood, also known as secondary xylem, is a composite of tissues found in trees. Secondary xylem is composed primarily of cells, called vessel elements in angiosperms, or of slightly different cells in gymnosperms called tracheids. These cells of secondary xylem, along with specialized cells of a type called parenchyma, are made by a meristematic tissue called the vascular cambium. As the vascular cambium generates new cells, secondary xylem accumulates on its inside, and the tree increases in diameter.
Newly made vessel elements and tracheids are water conduits from the roots of plants to their leaves. When first made, vessel elements and tracheids are alive but once they mature and become functional, they die. The functional vessel elements or tracheids occur in a few cell layers behind the vascular cambium, in a water-conducting section of the secondary xylem known as sapwood.
The parenchyma are made by the vascular cambium along with the vessels or tracheids, and are located at certain points along the perimeter of the vascular cambium. As the tree expands through growth, these narrow columns of parenchyma cells, called xylem rays, become longer, and ultimately extend from the vascular cambium to very near the center of the tree trunk. The function of xylem rays is to transfer aqueous material horizontally along the diameter of the tree, at a right angle to the flow of water in vessel elements and tracheids. The parenchyma cells of the xylem rays are alive in their mature, functional state.
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