Using mathematical equations, a Cornell University scientist and his colleagues have found evidence of a fourth spatial dimension in plants.

In short, size matters even in the plant world, suggesting that "universal scaling laws probably exist," says Karl J. Niklas, the Liberty Hyde Bailey professor of plant biology at Cornell. In the animal realm, the laws of scaling have been well known for more than a century. Yet only recently have plant biologists become aware of these laws' importance throughout nature.

"You've seen monster movies with giant ants scaled up in a huge size attacking cars and people. If these larger-than-life ants had the same proportions as their much smaller counterparts, their legs would break with the first step. To be much larger than life-size, an ant's exoskeleton would have to be disproportionately much thicker compared to that of a smaller ant," says Niklas. "For the same reason, if giant humans were scaled up as they are in some movies, their hearts wouldn't be able to circulate blood properly and they would die of a heart attack." This scaling is known as the fourth spatial dimension because it relates mass to the other three dimensions, width, length and depth.

Now, Niklas and his colleagues have shown that the same scaling laws known for animals apply equally to plant life, including trees. Niklas and co-author Brian J. Enquist, an assistant professor in ecology and evolutionary biology at the University of Arizona, Tucson, present their findings in a paper, "Invariant scaling relationships for interspecific plant biomass production rates and body size," in the online edition of the Proceedings of the National Academy of Sciences (PNAS) . It will be published in a forthcoming issue of the journal PNAS .

The finding could have profound effects on environmental and ecological policy, as well as the science of evolutionary biology. In the future, plant scientists will have the ability to develop mathematical models to make predictions in such areas as standing forest biomass and growth.

In their paper, Niklas and Enquist show that plant growth increases at three-fourths the rate of plant body mass, the same scaling relationship as for animals. For example, as a redwood tree grows in size over centuries, its rate of growth gradually slows down according to this very precise mathematical relationship. This relationship was first predicted by Enquist, Geoffrey B. West of the Los Alamos National Laboratory, Los Alamos, N.M., and James H. Brown of the University of New Mexico, Albuquerque, N.M.

The authors of the PNAS paper suspect that their finding in plant biology has applied since life began on earth. "Because present-day plants and animals appear to abide by the same or very similar scaling rules, there is good reason to expect these rules extend into deep geological (evolutionary) time. … [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 researchers say in their report."

Says Niklas, "Our data say that growth rates are indifferent to other biological differences across species. In scaling, a tree is a tree is a tree."