Study results released Wednesday show surprising parallels between the "plumbing" systems of plants and people.
The research reveals the distribution of vital fluids through pipe-like structures of "fleshy" flora follows a law that for nearly 80 years has described how life-sustaining forces circulate through human veins and arteries.
The remarkable similarities in the way many plants transport water through their woody tissues, called xylem, and animals carry blood through their circulatory systems, hold significant implications that will rewrite biology textbooks, said physiologists who spent two years slicing and dicing thousands of leaves, stems, twigs and branches to cut through to the unexpected findings.
Since 15th century Italian maverick Leonardo da Vinci first proposed the notion, scientists have thought of xylem -- composed of thin, hollow tubes of dead cells that conduct water and nutrients from root to crown -- as a collection of pipes whose total number stays constant throughout the plant.
In an analysis to be published in the Feb. 27 issue of the British journal Nature, Katherine McCulloh and colleagues at the University of Utah in Salt Lake City have shaken up this long-held concept.
"Our work shows that the conventional view of xylem anatomy, which is that the number of xylem conduits or tubes stays the same from the trunk to the leaves, is wrong and that actually the number of conduits increases," said McCulloch, a biology doctoral student who shredded plant parts with a razor blade to dissect the mechanisms of floral water transport. "This may seem like a trivial difference, but the result is that plants are actually more efficient than previously believed," she told United Press International.
Indeed, the study suggests the waste-not-want-not formula that drives animal circulation applies to herbaceous plants as well -- despite profound differences in the two groups' substance, structure and style.
"This study shows that plants and animals have reached the same solution for moving fluid most efficiently despite their radically different vascular systems," said McCulloh, first author of the study.
As an example of the differences, blood flow exacts a price in the energy necessary to keep it coursing, while the movement of water in plants is free of such a fee, noted Fred Adler, associate professor of mathematics and biology and co-author of the study.
Water evaporates from leaves through a process called transpiration. Meanwhile replacement fluids are pulled up from the xylem, whose long, narrow cells -- stacked like so many concrete sewer pipes -- serve as a conduit to haul water from the roots.
Because they are no longer alive, xylem cells cannot participate in active pumping as their circulatory counterparts do. Rather, the water is sucked up, like a drink through a straw. The action is made possible by the water molecules' unique dual ability to stick to one another as well as to many surfaces. These two features allow water to be pulled like a rubber band up the small xylem tubes.
"The major limitation to the plant system is that the pipes cannot become too few or too large or the system will become overly vulnerable to failure," explained John Sperry, professor of biology and co-author of the study.
"The xylem water is under negative pressure -- hence any leak will allow air to rush into the pipes and block flow. There must be enough pipes present to provide redundancy against this problem," he told UPI. "In animals, the blood is under positive pressure, and leaks are stopped by blood clotting. There is less of a necessity for having multiple
piping systems running in parallel."
Despite the differences, the study suggests both systems subscribe to a law devised by British biologist Cecil Murray, which predicts the manner in which arteries, vessels and capillaries should taper to progressively smaller diameters to optimize the efficiency of transporting blood through the body with minimal friction.
"In both transport systems, fluid journeys from a few large-diameter pipes through more progressively smaller-diameter pipes as the system branches to deliver fluid to the tissue," Sperry told UPI. "This kind of structure minimizes the frictional resistance to flow, even though the pressures and mechanism of flow are quite different."
The study is the first to note this similarity.
"Until this study, there had not been a serious attempt to extend the theory of Murray's law to the xylem of plants," McCulloh noted. "In fact, some of the textbooks that discuss Murray's law are quick to point out the reasons it should not apply."
Murray devised his eponymous law in 1926 to describe the structure of animal arteries and vessels.
"The neat thing about this paper is that in the almost 80 years Murray's law has existed, no one until Kate (McCulloh) seriously considered applying it to plants let alone working out the necessary extensions to the law, then taking it a step further and testing how well it actually describes the plants' vascular system," Sperry said.
Noted biophysicist and virologist William Lucas, professor of plant cell biology, genetics and physiology at the University of California, Davis, deemed it an elegant, well-conducted study. He said it portended a new frontier in biology, carrying wide-ranging implications, from future bioengineering of wood to a clearer understanding of climate change.
"I think these are quite interesting findings on a fascinating topic," Lucas told UPI in a telephone interview.
Murray's law allows a plant to minimize frictional resistance to fluid flow in its vascular system, which frees up more of an organism's resources for other functions -- most important, reproduction, Sperry said.
"If you are an engineer building a plumbing system, you want to deliver the most water per unit of energy with the least amount of material -- the cheapest and most effective conductance system you can design," he said.
Plants marching to Murray's law include all "fleshy" flora, such as wildflowers, grasses, vines and palms, and the leaves and root systems of all plants, Sperry said.
Those that do not include the trunks of pines, firs, redwoods and other conifers and of maples, birches, cottonwoods and other "diffuse-porous" trees, he added.
The law, which shows the total cross-sectional area of the piping system increases from stem to leaves along with the total number of conduits, contradicts Leonardo's constant-area view and the popular "pipe model" of vascular transport it inspired, Sperry said.
Leonardo, a Renaissance artist, scientist, engineer, architect and inventor, proposed, "if you take a tree, the cross section of the trunk should be equal to all the (cross sections of) branches above that trunk added together."
The ensuing pipe model portrayed plant water conduits clumped together like pipe cleaners that bend apart to form branches.
In fact, the conduits in plant xylem resemble numerous short pipes, paralleling each other, with their ends offset, McCulloh said. Instead of flowing through a continuous conduit, water sloshes through individual vessels, one at a time, she said.
In the two years of slicing thin plant cross sections with a razor, then analyzing them under a microscope, McCulloh measured some 100,000 water-bearing conduits in leaves, twigs, stems and branches from ash and box elder trees and Virginia and trumpet creeper vines. In a single tree, she measured the diameters of 22,000 conduits at various levels, from branches to leaves.
"The chief challenges were Kate's -- making those 100,000 slices and measuring them carefully without going insane," Sperry said. "We also had to develop some novel statistical methods to analyze the results."
The payoff was well worth the effort, said the scientists, who envision using the information in future efforts to bioengineer lumber or look closer into climate-regulating carbon cycles.
"Kate's project vastly improves our understanding of how water moves through plants," Sperry said. "We're creating information that goes into new editions of biology textbooks. It is increasing knowledge of the natural world."
