100x Magnification

In this 100x magnification locate a vertical line that appears to be separating two distinct areas of xylem. Note how cells to the left of this line are larger than those immediately to the right. This is a transition zone between two years of successive growth. The wood to the right is denser summer wood while that to the left is more open spring wood. In woody species whose early xylem possesses more large vessels, a ring porous type of wood forms. Diffuse porous woods are found in species that form large vessels throughout the growing season.

200x Magnification

In the 200x view the size difference between vessels of spring and summer woods is pretty obvious now. Note how the majority of xylem cells possess thickened walls and appear empty. Remember, these are sclerenchyma cell types that are dead at maturity.

Now, find the horizontal lines of living parenchyma cells that form the vascular rays within the wood. Vascular ray cells provide lateral transport of sugars and nutrients within stems. These cells can also divide mitotically to form new xylem to either side as the girth of the tree expands during growth.

400x Magnification

A 400x view of this wood is now clearly showing the radial lines of living parenchyma cells. Once again, try to envision these cells involved in lateral transport of metabolites through the stem.

Compare this wood to that of a conifer now by resting your mouse on the green microscope light. Wood of conifers is made of tracheids only and these are narrower than vessels. Find the transition between spring and summer woods. Also note the resin canal to the right, a typical conifer structure.

1000x Magnification

At 1000x you should contrast the living parenchyma cells with vessels of the xylem. Lignified secondary walls of xylem cells contribute to the strength of wood and these tubular cells conduct water quite well. However, vessels and tracheids of wood are dead at maturity and therefore cannot serve as a source of new cells. Living parenchyma cells of vascular rays and the vascular cambium serve as sources of new cells.

Microscopy of a Woody Stem

Woody stems and roots produce xylem yearly. The xylem accumulates as successive layers or growth rings. Three are visible in this woody Tilia stem.

The boundary between phloem and xylem is the vascular cambium, a secondary meristem. You can see it to the left in this image. Most tissue of a large tree is secondary xylem (wood) that accumulates inside this meristem. Phloem produced by this meristem is found just under the bark of trees.