Supply and demand

Trees need highly efficient channels, or ducts, for transporting water and nutrients from the soil, as well as the sugars produced by photosynthesis (Synthesis of organic substances made by plants from the light energy.). We humans have a heart to pump blood through our bodies. Trees don't have any muscles, though, and they have to fight gravity over a much larger distance than we do.

The xylem, hidden inside the sapwood, carries water and nutrients from the tree's roots to its foliage. The phloem redistributes the sugars from the leaves to the parts of the tree that need them. This two-way traffic is governed by a very simple imperative: to get the resources from the supplier to the consumer.