**1. Transport Systems**
There is a piping system which sources from the root, and reaches every cell in the leaf.
A pipe from cell A to cell B may be viewed as an edge (A,B) in a directed graph.
But it's not just any kind of graph, it's a tree which is rooted at the root cell.
Every cell (except for the root) has a single parent -- this is the predecessor cell which supplies it with fluid.
And every cell has a set of children, which are cells that it directly pipes fluid to.
A cell which has zero children is at the frontier of the graph.