One of the biggest mysteries about living things is whygiven their vast diversitythey should all be built according to a simple rule known as quarter-power scaling. In short, this rule sees to it that many of an organism's traitsfrom average life span to number of offspringvary according to the animal's mass (M) raised to some multiple of one-fourth. In both a mouse and a blue whale, for example, respiratory rate is proportional to M-1/4, and metabolic rate scales with M3/4. Because the scaling relates mass to the other three dimensionswidth, length and depthit is referred to as a fourth.

Scientists have studied this seemingly universal scaling law in animals for more than a century, but now they have discovered that it extends to plants as well. In this week's issue of the Proceedings of the National Academy of Sciences, Karl J. Niklas of Cornell University and Brian J. Enquist of the University of Arizona report that growth rates, body lengths and pigment concentrations in a wide range of flora follow quarter-power scaling.

The group examined both unicellular and multicelllular plants, from marine algae to conifer trees, and found that annualized rates of growth scale as the 3/4-power of body mass, as does pigment concentration. So too, they discovered that a plant's body length scales as M 1/4. "[this] provides a potentially powerful tool for predicting many important properties for past as well as present day organisms and the communities in which they live," the authors write. "Our data say that growth rates are indifferent to other biological differences across species," Niklas adds. "In scaling, a tree is a tree is a tree."