Where the Tree Is Alive
This section of the trunk of a small tree is pictured in such a way as to expose the Cambium layer, just beneath the outer bark. In this layer are located all the protoplasmic cells, aside from those in the leaves, that are really alive. A portion of the woody tissue just beneath the Cambium conveys the watery solution upward from the roots; but the return flow of sugary sap takes place solely in the Cambium layer, where also the protoplasmic or life activities go on, through which the tree grows; growth itself being due to the deposit of what is virtually waste material from the cell. The central wood fibers of the trunk are totally dead.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 7 Chapter 1