Stiff as a board
Although you might think that the tree would have a living core protected deep inside it, curiously enough that isn't true. Its very centre is made of dead wood, or heartwood, whose main role is to make the tree rigid and stable.
That being said, this heartwood isn't actually essential. Some trees are attacked by parasitic fungi and become completely hollow. They remain standing even after their heartwood has been digested by the fungi. The tree chooses to defend its living tissues, and tries to confine the invaders to the dead wood at its core.
While the tree is forming its heartwood, or duramen, in a process aptly known as duraminization (Lignification of wood cell membranes.), it stores different molecules in this dead wood. These molecules give the wood its characteristic natural colour, smell and durability. Some of them are even toxic, to protect the heartwood against invaders.
