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.
The more I think on what xylem does, the more I think about the hidden nature of servant-hood. You see, the vital part of the plant - the part that carries exactly what the tree needs - is surrounded by the outer rings. Without dissecting, the xylem goes unnamed.
It's the same with those who serve quietly for the purpose of giving what others need - and not to bring attention to themselves. What they do is surrounded, drowned out, by what is popular and attractive. It's when our culture is dissected or burned down that the chaff blows away to reveal what lasts; the xylems among us stand up in times of dissection - recession, oppression, war - and not only continue to serve, but are recognized for who they've always been.
They're the center of the tree.