Trees, like all plants, have three basic organs: roots, stems, leaves. In turn, organs are composed of different tissues; and tissues are composed of different cells.
The root system is subterranean; it provides anchorage and absorbance of minerals and water. Trees have a taproot - a large vertical root, with numerous smaller roots branching off. Taproots often store food. Most absorption occurs near the root tips, where the surface area is increased by vast numbers of tiny root hairs. Some trees, such as mangroves or banyans, have adventitous roots growing upward from the soil or downward from the branches. At the ends of the roots, the root cap shields the root apical meristem, where cells divide and growth occurs.
The shoot system consists of stems and leaves. Stems alternate between nodes (where leaves are attached) and internodes (the spaces between leaves). Stems end in the terminal or apical bud, with additional axillary buds located in the angle between leaf and stem. Axillary buds usually only grow if the apical bud is removed. Both apical and axillary buds contain meristems, but usually only the apical shoot meristem is dividing and growing. Leaves consist of a large flat area (blade) and a short stalk (petiole) which attaches the blade to the stem. Within the leaf is a branched network of veins. Some leaves are divided into smaller leaflets; these are called compound leaves.
Roots, stems, and leaves are all composed of three types of tissue: dermal, vascular, and ground. Dermal tissue, or the epidermis, is a layer of tightly-packed cells that covers the young parts of the plant. At the tips of roots, the dermal cells expand into root hairs. In leaves, they secrete a waxy coating (cuticle) to help the plant retain water. Specialised leaf epidermis cells open and close the stomata, the pores used for respiration.
The vascular tissue transports water, minerals, and sugars around the plant. Xylem carries water and dissolved nutrients upward from the roots to the rest of the tree. Phloem trnsports food made in the leaves by photosynthesis to the non-photosynthetic regions of the tree. Xylem cells - tracheids and vessel elements - are dead, elongated cells. Their cell walls contain thin areas called pits, which allow water to move from cell to cell. Phloem is carried by sieve tube members, alive due to the support of a companion cell. Vascular tissue is found in bundles arranged in a ring around the middle of the trunk or branch, with xylem vessels on the inside and phloem vessels on the outside.
All other parts of the tree are classed as ground tissue. That which lies within the ring of vascular bundles is the pith; that which is outside the vascular tissue is the cortex. Ground tissue functions in photosynthesis, storage, and support.
Plant tissues consist of three basic cells types: parenchyma, collenchyma, and schlerenchyma. Parenchyma are the least specialised plant cells. They perform most of the tree's metabolic functions, including photosynthesis and storage. All plant cells begin life as parenchyma cells. Collenchyma cells have thicker, more uneven walls than parenchyma. They are grouped into bundles wich help support young, growing parts of the tree. Schlerencyma cells are strengthened by the presence of lignin in their cell walls. They are dead at functional maturity and serve as support for the plant. Xylem cells are schlerenchyma.
Height is increased by primary growth, but girth is increased by secondary growth. Within plant stems is a cylinder of meristematic tissue called the vascular cambium, which runs through the bundles of vascular tissue; this contains cells which divide to form secondary vascular tissue (xylem and phloem). Over time, the accumulation of secondary vascular tissue produces a thickening of the stems, forming the trunk and branches. What we call wood consists mostly of secondary xylem with thick, lignified walls. Early in secondary growth, the epidermis splits and falls away. This is replaced by new tissue produced by the cork cambium, another ring of meristematic cells. This produces cork cells, which accumulates outside the cmium. These deposit waxy suberin, which protects the tree from damage and pathogen attack, and water loss. The cork and the cork cambium are collectively known as the periderm; together with the secondary phloem, they form what is commonly known as bark.