Phloem, Cambium and Xylem

• Just inside the hard outer layer of dead cells that form the tree's outer bark is a layer of cells called the phloem or inner bark. These are tubular cells that move nutrients created in the leaves and dissolved in water to other parts of the tree. Inside the phloem layer is a layer of living cells called the cambium. The cambium is responsible for the growth of the trunk, forming new cells that grow both outward and inward, renewing the cambium and the wood at the center of the trunk. The wood cells are called xylem, and they function as a transport system for water from the roots to the rest of the tree and as storage for important chemical compounds.
  
  

Sapwood and Heartwood

• The woody part of an oak tree's trunk is made up of two types of wood. The outer layer of wood, called sapwood, is made up of living cells. As sapwood ages and is replaced by new cells, it is less involved in the vital functions of the tree, and the cells gradually die. This dead wood that forms the center of the trunk is called heartwood. It serves as support for the tree and as a storage area for chemicals that help the tree resist damage from bacteria, fungus and insects.
  
  

Roots

• The tips of the roots contain a collection of cells called the apical meristem. These cells divide and reproduce, forming specialized root cells and causing the roots to grow in length. The meristem is covered by a protective layer called the root cap. Specialized root structures called absorbing roots take in water and nutrients and channel them to conducting roots, which transport them to the tree's trunk.
  
  

Leaves

• Leaves are made up of spongy cells that contain chlorophyll and convert sunlight into nutrients that can be used by the tree. Leaves take in air via openings in the leaf surface called stomates, and nutrients are transmitted from the leaf cells to the vascular system of the branches by tubular conducting cells.