I am fascinated by the structure of a tree - and this may be cheating to talk about trees again, but I can't help it.
Xylem is basically wood, however, it's the part of the tree (or plant) that does the heavy lifting. Through the xylem, and the phloem, water is carried to branches and leaves.
Without this part of the tree, well, there is no tree. The xylem is why it seems trees can defy gravity by pulling water up from the ground. I remember doing science experiments when I was first learning about plants; we used dye and a young translucent plant so we could watch the water travel in what seemed like an impossible way.