Anatomy of the Woody Tree Trunk

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Woody tree trunks exhibit a complex internal structure that reflects their functions in transport, support, storage, and long-term growth. Unlike herbaceous stems, tree trunks undergo secondary growth, resulting in an increase in diameter over time. This growth is produced by lateral meristems and gives rise to the characteristic tissues of wood and bark.

External and Protective Tissues

The outermost region of a tree trunk is collectively referred to as the bark, which includes all tissues external to the vascular cambium. Bark serves as a protective barrier against physical damage, desiccation, temperature extremes, and biological threats such as insects and pathogens. In younger stems, the bark may include remnants of the epidermis and cortex, although these tissues are largely replaced as secondary growth progresses.

Vascular Tissues and Secondary Growth:

Phloem: Located just beneath the bark, the phloem is responsible for the transport of organic compounds, primarily sugars produced during photosynthesis, from the leaves to other parts of the plant. Phloem tissue is living and consists of specialized cells adapted for long-distance transport and metabolic support.

Vascular Cambium: The vascular cambium is a thin, continuous layer of meristematic cells situated between the phloem and xylem. It functions as a lateral meristem, producing new vascular tissues through cell division. Cells produced toward the outside differentiate into secondary phloem, while those produced toward the inside become secondary xylem. The activity of the cambium is responsible for secondary growth, increasing the girth of the trunk and forming annual growth rings.

Xylem: The xylem constitutes the majority of the tree trunk and performs multiple essential functions. It conducts water and dissolved mineral nutrients from the roots to the leaves, provides mechanical support, stores carbohydrate reserves, and contributes to defense against decay. Most xylem cells are dead at maturity, forming a rigid framework that supports the tree.

As xylem ages, its innermost layers become non-conductive and form heartwood. Heartwood does not participate in water transport but plays a critical role in structural support and long-term durability. It often accumulates chemical compounds that inhibit the growth of fungi and other decomposers, thereby enhancing resistance to decay.

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Living and Non-Living Tissues

Within the trunk, tissues can be categorized as either living or non-living. The symplasm refers to the interconnected living components of plant tissue, including living xylem parenchyma cells. These cells are involved in storage, defense, and internal transport. In contrast, the apoplasm consists of non-living structures, such as dead xylem cells, through which water can move passively.

Parenchyma cells within the xylem remain alive and play important roles in storing sugars, contributing to defense mechanisms, and providing structural continuity across the xylem through ray tissue.

Xylem Cell Types and Evolutionary Differences

Gymnosperm Xylem: In gymnosperms, such as conifers, water conduction and mechanical support are carried out almost exclusively by tracheids. Tracheids are elongated, dead cells with tapered ends that allow water to move between cells through pits in the cell walls. Although less efficient at water transport than vessels, tracheids provide significant mechanical strength.

Angiosperm Xylem: Angiosperms possess a more complex xylem structure. Their primary conducting elements are vessels, which are composed of stacked vessel elements forming long, continuous tubes. These hollow structures allow for highly efficient water transport. Angiosperms also retain tracheids, giving them both conductive efficiency and mechanical support.

Wood Porosity Patterns

The distribution of vessels within angiosperm xylem varies among species and is an important characteristic for wood and tree identification.

In ring-porous species, vessels formed early in the growing season (earlywood) are large, while those formed later (latewood) are significantly smaller. This creates distinct growth rings. Common ring-porous trees include elm, oak, and ash.

In diffuse-porous species, vessels are relatively uniform in size throughout the growing season. Growth rings are less distinct, and water conduction occurs more evenly across the xylem. Examples include maple, plane tree, and poplar.