Pith is a tissue found at the very center of a tree’s stem, representing the initial core from which the rest of the wood develops. While it plays a role in the tree’s early growth, its significance changes considerably as the tree matures and wood forms around this central point.
Understanding Pith’s Structure and Location
Pith is composed of soft, spongy parenchyma cells, characterized by their relatively large size and thin cell walls. When newly formed, pith typically appears white or pale, but it commonly darkens to a deeper brown as it ages.
It is located at the innermost core of the tree stem, surrounded by rings of xylem (wood) and phloem. In dicotyledonous plants, such as most trees, the pith is distinctly central, while in monocotyledons, its arrangement can differ.
The appearance of pith can also vary significantly between different tree species. For instance, some trees, like walnuts, feature a distinctive chambered pith with numerous short cavities, while oaks can have a star-shaped pith when viewed in cross-section. Other species might have a solid pith, or it may even dry out and disintegrate, resulting in a hollow stem. This variation can be used for identifying certain tree species, particularly in winter when leaves are absent.
Pith’s Function in Tree Development
Pith’s function is primarily observed in the early stages of a tree’s life, especially in young shoots and saplings. During this period, it serves as a storage site for nutrients, including starches, sugars, and water, which support the plant’s initial growth and elongation. It also plays a role in the lateral distribution of these substances within the stem.
As a tree undergoes secondary growth, forming new wood (xylem) layers, the pith’s active functions largely diminish. The surrounding wood tissue, which becomes more robust, takes over the roles of support and long-distance transport. The pith often becomes non-functional and is sometimes replaced by xylem or can even disintegrate, leaving a hollow space.
In mature trees, the pith is dead tissue, encased by the dense wood that has formed around it over decades. The cells within the pith can retain chlorophyll for several years, even after being surrounded by layers of wood.
Practical Implications of Pith in Wood
The presence of pith in sawn timber has several practical considerations. Pith is the most unstable part of a board and has low strength compared to the surrounding wood.
Wood containing pith is particularly prone to cracking and splitting as it dries. This occurs because wood shrinks differently in various directions—tangentially (around the growth rings) versus radially (from the pith to the bark). Since the pith is the convergence point for all these planes of shrinkage, it experiences significant stress, often resulting in checks or cracks radiating outward.
The “juvenile wood” immediately surrounding the pith, which formed when the tree was young, also contributes to instability. This wood has different shrinkage characteristics and can cause boards to bow or distort.
For these reasons, pith is frequently avoided in high-quality lumber intended for applications requiring stability and durability. While large slabs or certain dimensional lumber might still contain the pith, its presence often necessitates specific drying techniques or design considerations to mitigate cracking and warping. Woodturners, for example, often remove the pith from blanks to prevent splitting during the drying process. Cracks around the pith may be filled with epoxy or incorporated as a design feature.