Vascular Cambium and Rays

On this section of wood, a small portion of the vascular cambium is highlighted in red. Find this vascular cambium and use its location to orient yourself to the features of this piece of wood.

The vascular cambium generates layers of secondary xylem toward the inside of the vascular cambium "circle." All of the tissue within the vascular cambium is secondary xylem, also called wood. Since the vascular cambium generates a new layer of secondary xylem each year, and since early-spring wood (carrying lots of water) consists of cells with larger lumens while the late-summer wood has cells with smaller lumens, secondary xylem is laid down in growth rings. Look at the secondary xylem closely. Try to distinguish the larger lumens of the spring wood from the denser summer wood in one growth ring.

Also notice the many lines radiating from the center of the tree, through the xylem. These are rays, aligned parenchyma that function in transport and storage. Rays also help to allow wood to expand without cracking. When the tree dies and the wood dries, these rays dry and the wood often cracks along these rays. (These cracks provide convenient fracture lines for splitting by axe or wedge.)

Now examine the tissues outside of the vascular cambium. Notice that this tissue has two different layers: a lighter layer right next to the vascular cambium, and a darker layer that is the outermost tissue of the tree. The lighter layer, just outside the vascular cambium, is secondary phloem generated by the vascular cambium. Of this secondary phloem, only the current year's growth (nearest the vascular cambium) is living tissue in a live tree. The secondary phloem is called inner bark. The darker, outermost layer of tissue is periderm, or outer bark, a protective layer for the tree. In this tree section, it is not easy to distinguish the cork cambia within the periderm. Together, the inner bark (secondary phloem) and the outer bark (periderm) are referred to as bark.

Examine some of the smaller chunks of wood in the Study Center and try to identify rays, secondary xylem, the area of the vascular cambium, secondary phloem, and periderm.

Back to Main Menu / Previous Objective / Next Objective /