The trunk of a tree is the conduit through which water and nutrients absorbed by the roots pass up to the leaves to facilitate photosynthesis. The five parts of the trunk provide the tree with both a sturdy backbone and a transport system for food produced in its leaves with the help of the sun's energy. Together, the parts of the trunk give a tree stability, support for a heavy crown of branches and a means to transport food and waste products from roots to leaves.
The outermost layer of a tree trunk is the bark. Bark consists of dead cells that form a thin but hardy coating to protect the inner parts of the trunk from damage by weather, infection and infestation by insects and other animals. The bark of each species of tree has its own characteristics; the texture, color and thickness of the bark can be used in identifying the species of the tree.
Phloem, the Supply Line
Inside the bark lies the phloem, the vascular system of the tree trunk. Phloem is living tissue that transports the sugary sap produced in photosynthesis. The sap moves down the phloem through sieve-tube cells and transfer cells, allowing the food made by the leaves to feed the other parts of the tree.
Cambium, the Growth Rings
The vascular cambium is the area of growth in a tree trunk. The cambium is living tissue that can produce either new phloem tissue or new xylem tissue. The tree grows outward in seasonal rings from tissue produced by the cambium.
The two-part xylem makes up the inner core of the tree trunk. The dead cells of the xylem form the bulk of the tree trunk. Sapwood is the outer layer of xylem. This younger tissue transports water and nutrients that have been absorbed by the roots up the tree trunk to the crown.
Inside the sapwood is the heartwood. Heartwood is older xylem tissue that forms the core of the tree trunk. Heartwood is largely inactive in the transport of materials throughout the tree.
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