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.
Illustration courtesy University of Florida