Wood is a highly ordered arrangement of living, dying and dead cells. These tree cells function much like a lamp wick where the tree is anchored. The roots are bathed in a nutrient rich liquid which transports these nutrients plus moisture to the top where all is consumed.
A tree (and the cells) supports an ever-flowing wet system that must be maintained at all times. If the process fails to provide water at any point the tree will eventually die due to the failure of both water and food requirements that are necessary for life. Here is a biology lesson on tree cells.
Images used are provided by the University of Florida, Landscaping Department.
1. A Tree's Cambium
The cambium and its "zone" is a cell generator (reproductive tissue called growth meristem) that produces both the inner bark cells of the phloem and new living wood cells in the xylem. The phloem transports sugars from leaves to roots. The xylem is a transport tissue and both stores starch and conducts water and substances dissolved in water to leaves.
2. Phloem, A Tree's Inner Bark
Phloem, or inner bark, develops from the outside layer of the cambium and is the food track to the roots. Sugars are transported from leaves toward roots in the phloem. When the tree is healthy and growing and sugars are abundant, stored food in the form of starch can be converted back into sugars and moved to where it is needed in the tree.
3. Xylem, A Tree's Nutrient Transport System
Xylem is living "sapwood" and located inside the cambial zone. The outer portion of xylem is conducting and storing starch in the symplast plus conducts water and substances dissolved in water to the leaves. The inner portion of the xylem is non-conducting wood that stores starch and is sometimes called heartwood. The major structures for water transport in xylem are vessels in angiosperms (hardwoods) and tracheids in gymnosperms (conifers).
4. Symplast, A Tree's Storage Network
Symplast is the network of living cells and the connections between living cells. Starch is stored in the symplast. Axial parenchyma, ray parenchyma, sieve tubes, companion cells, cork cambium, the cambium, and plasmodesmada make up the symplast.
5. Vessels and Tracheids, A Tree's Conductors
Vessels (in hardwoods) and tracheids (in conifers) conduct water and substances dissolved in water. Vessels are vertically aligned tubes made up of dead cells that transport liquid. Vessels are found only in angiosperms. Tracheids are dead, single-celled "pipes" that act much like vessels but are only found in gymnosperms.