by Elizabeth Hedger

Created on: April 06, 2007   Last Updated: April 18, 2007

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

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