Background Information on wood cells, vascular tissue and maple sap


Introduction

Wood is composed of plant cells that are surrounded by cell walls. The thickness or density of cell walls is an important property that determines the uses and economic importance of wood. All trees are vascular plants, which means they have cells that conduct fluids. There are three main types of vascular tissue in trees: xylem, phloem and rays. Students can observe these cells and tissues when looking at paper products, blocks of wood, or tapping a maple tree. Your 4-H Extension Agent has lessons on Virginia Natural Resources and tapping a maple tree that can help you teach these concepts.

Wood cells   to the top

Plant cells are surrounded by cell walls. The thickness of cell walls in a tree’s trunk determines the density of its wood. High-density woods such as oak and hickory are used to manufacture products that are durable and resist wear, such as flooring, railroad ties, and furniture. Low-density woods such as the conifers, maples and tulip tree are used to manufacture products that are easily worked, such as construction lumber, oriented strand board, furniture and cabinet parts.

Wood cells also differ by length and shape. Pine fibers are long and narrow, making them ideal for the paper making process. They are generally the fibers that hold a piece of paper together. Hardwood fibers are highly variable and have varying qualities that are often mixed with pine fibers to make various paper products.

To view pine cells, hold a piece of toilet tissue up to the light and note the fibers that are visible to the naked eye. These are wood cells taken from the trunk of a tree.

To view large hardwood vessels, look at the end of a piece of oak lumber or cookie to see the large, water conducting cells that look like open-ended tubes or “pipes.” These are the annual rings of oak trees. (Note: you may need to slice the edge of your wood sample with a razor blade so that the tubes are clearly visible.)

Vascular tissue   to the top

There are two primary types of vascular tissue in trees, the phloem and the xylem. Both are located in the trunk of the tree and consist of dead cells. They can be thought of as “pipes” that conduct fluids between the tree’s crown and roots. Xylem tissue conducts water up the tree from the roots to the crown and is also known as sapwood and heartwood. Phloem conducts food (sugars, starch) down the tree from the crown to the roots. It is located just underneath the tree’s bark.

Rays are another type of vascular tissue. Rays store sugar in the tree’s trunk, and conduct sugars between the phloem and the xylem. Rays are visible to the naked eye in the wood of oak trees. Their ability to transfer sugars between phloem and xylem is best known among maple trees.

Oak cookies have highly visible rays and vessels, two of the most important structures for understanding sap flow. The “annual rings” in an oak tree are actually water conducting vessels in the xylem and can be seen with the naked eye or a hand lens. Rays are lines that move horizontally (across) the cookie from the phloem to the center.

Xylem tissue varies by species. Conifers (pines, spruces) have xylem tissue that is composed of long, narrow cells called tracheids. In springtime, conifers produce tracheids with larger openings that are lighter in color compared to summertime growth. Annual “rings” are actually boundaries between spring and summer growth.

Hardwood xylem contains specialized cells called vessel elements. These cells are generally much wider than the tracheids of conifers. Ring-porous hardwoods, such as an oak, produce an annual “ring” of very wide vessel cells in the springtime. Other hardwoods, including maple trees, are diffuse-porous, with wide vessel cells appearing throughout the xylem. In either case, the annual “ring” is actually a boundary between spring and summer growth. In some species, the boundary is not distinct, making it difficult to age.

The pictures on the student handout represent an attempt to represent generalized wood structure. Consequently, the longitudinal cells pictured in the magnified section would be tracheids in a conifer, and vessel cells in a hardwood.

Maple Sap   to the top

The sugar in maple sap is sucrose from the previous year’s photosynthesis. In wintertime, sugar is stored in ray cells. Hardwood trees have contact cells (not visible) in the xylem which allow sugar to move from rays to the vessels, so that sugar can be delivered to the buds where it will be needed to energize growth. Maple trees have guard cells that are unusually good at this, making them unique among trees.

Sap flows from a maple tree when sap pressure inside the tree is greater than atmospheric pressure. When you drill a tap hole, you create a release for that pressure. Sap pressure is greatest on warm days that follow cool nights. Sap pressure is lowest at night.

So what causes sap pressure? On cold nights, a cooling sequence involving contraction of CO2 in sap, and ice-induced absorption, causes water to be drawn up and into twig fibers in the crown of the tree. When it warms the following day, the reverse occurs as the gas expands into the recharged fibers. This forces water to move out and down the tree, causing the greatest pressure at its base, where your tap is located.

 


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