Background Essay: Shape of Things: Trees
All organisms are made up of cells. And nearly every cell, whether it comes from a plant or an animal, a petunia or a whale, is more or less the same size--about 10 to 30 micrometers across, or about one-tenth the diameter of a human hair.
This similarity in size among plant and animal cells is no accident. Active cells really can't be any bigger than they are because they must be able to exchange nutrients and gases across their membranes. If a cell is too large, the ratio of its surface area to its volume will be too low for it to efficiently take in what it needs and get rid of what it doesn't.
Another striking similarity between plants and animals is the manner in which nutrients are distributed to the cells. All organisms rely on a branching system of tubes that puts every cell in contact with every other cell and, more importantly, with the source or sources of the nutrients they need. For example, the 10 trillion cells in the human body are fed regularly by a cardiovascular system that includes some 10 billion capillaries.
Scientists refer to these systems as "fractal branching networks," whether they are the vascular systems that serve plants or the circulatory systems that distribute nutrients to animal cells. This means that any given portion of such a system is essentially identical, except in scale, to any other portion of the system. A capillary network in muscle tissue, for example, is simply a miniature version of an artery network in the chest. In essence, every energy transport system is like a tree, with branches of the tree differing in size depending on their location.
Remarkably, in most branches of any one of these systems, the velocity and pressure of the fluid inside the tubes remains constant regardless of its location or the number of times the tubes have branched. This is because the combined cross-sectional area of the tubes at a given branching level is equal to the cross-sectional area of the larger tube from which they feed.