"The trunk and branches support the weight of the tree and are the main elements in building a [...] shape in the plant.  The skin protects the interior part of the plant; the vessels provide channels for the passage of nutrition to the leaves and branches.  These vessels are the means by which water and nutrition absorbed by roots are passed to the leaves." —Thank you, Amy Liang, for The Living Art of Bonsai, 2005, p.171.