SAPWOOD: CHARACTERISTICS, FUNCTIONS, EXAMPLES - BIOLOGY - 2025

The sapwood, also known as "living wood" is the portion of the physiologically active wood xylem which is responsible for the transport of substances to the whole body of the plant. In addition to its driving activity, it is characterized by being light in color. Participates in the support and storage of substances.

The opposite term is the fraction of the wood whose vessels have been plugged and no longer exhibit conductive activity. This wood is darker and is called heartwood. When the sapwood dies, it becomes heartwood.

Sapwood is the lightest portion of the growth rings. Source: pixabay.com

Wood in general is characterized by having a very low water content. Sapwood can be made up of 35 to 75% water, while in heartwood the amount of water is reduced somewhat.

When we observe a cut trunk in its cross section, we will observe growth rings that correspond to changes between the heartwood (inner rings) and the sapwood (outer rings).

characteristics

In plant biology, the term sapwood refers to the young and active portion of the wood.

When we make a cross section of a tree, we will clearly see the famous growth rings. The sapwood corresponds to the last rings that we observe, and that generally exhibit a lighter color. It originates from the vascular cambium.

Plants have a series of tissues, and one of these forms the structures responsible for orchestrating the process of conducting water, salts, nutrients and sap, among others. The conducting structures of plants are the xylem and phloem.

Specifically, the xylem is responsible for the transport of water and minerals. In terms of structure, it is made up of tracheal elements, called tracheids and tracheas. Sapwood corresponds to the only xylem that functions in the trunk of the organism.

Differences between sapwood and heartwood

A related term is the heartwood, which corresponds to the darkest portion of the growth rings. Unlike the sapwood, the heartwood does not carry water and other substances: physiologically it is inactive.

Why is this happening? Throughout the years, the xylem rings are responsible for conducting the sap. However, with the passage of time, the conduction of water within these biological columns is interrupted by a phenomenon called cavitation.

Cavitation, also known as embolism, occurs when the continuity of the water column is broken due to the formation of gas bubbles. This is analogous to the embolism process that occurs in the blood vessels.

Thus, the ability to drive in the xylem is lost and this area, which was previously sapwood, becomes heartwood. Due to the accumulation of resins and gums, it acquires a darker hue than sapwood, which is usually white or light. Logically, the plant still needs a conductive surface, an activity carried out by the sapwood.

Why doesn't the heartwood decompose?

Now, if the heartwood is dead tissue, the plant must have mechanisms that prevent the decomposition of said structure.

To prevent the heartwood from starting a decomposition process and to prevent attack by fungi or any other pathogen after cavitation, the interior of the vessels is blocked by tyloses.

Tyloses are extensions of the cell cytoplasm that cover the lumen of the vessel. These cell processes come from the parenchyma cells. In addition, the process is accompanied by secretions against fungi and bacteria that protect the wood. These substances give the wood its unique smell.

Sapwood to heartwood ratio

If we study the cross sections of logs in different plant species, we can conclude that the proportions of conductive wood versus non-conductive wood vary widely.

In some species we will find a large amount of sapwood, such as maple, birch and ash. In contrast, other species exhibit very fine sapwood, as in the false acacia, catalpa, and yew.

On the other hand, there are certain species where there is no notable distinction between the sapwood portion and the heartwood portion that make up the wood. Examples of this are poplars, willows, and firs.

Sapwood functions

As we mentioned, sapwood is that clear portion of the wood that is responsible for the conduction of water and minerals, since it is the only area where we find the xylem elements active. Driving allows these vital materials to be carried to all regions of the tree.

In addition, the wood of a tree gives it rigidity, adding a supporting function to vascular plants. This is considered a general function of the xylem. Finally, it participates in the storage of reserve substances.

Example: secondary growth in gymnosperms

To exemplify the distribution of albumin in plants we will use a specific example of the genus Pinus, a well-known gymnosperm (we use a single species and do not generalize since the distribution and quantity of sapwood vary widely).

In the closed cylinder model of gymnosperms, the cambium is responsible for the proliferation of the vascular bundles: the xylem towards and inside and the phloem towards the outside.

Thanks to this distribution, the spring growth ring pattern is built, made up of thick, light-colored elements, followed by summer growth, where the elements decrease in diameter but increase the thickness of the walls.

In this case, the sapwood includes parenchymal cells, the protoplasm of which is alive. In contrast, the cellular elements of the heartwood are dead, and they store tannins and all kinds of resins that give the area a dark coloration.

Sapwood in the industry

It is widely known that wood is of great economic importance. A useful and easy way to infer the quality of the wood is by observing the sapwood and heartwood.

If the wood is rich in sapwood, its value decreases and it is not so appreciated by merchants and carpenters, since it has been shown that this area of ​​the wood is prone to rapid rotting and does not have the necessary characteristics to work on it effectively.

References

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