Why don't trees "bleed" to death when they are injured? Researchers from Virginia Tech, the Georg-August University of Gottingen, Germany, and the Jackson Laboratory of Bar Harbor, Maine, have discovered how "check valves（止回阀）" in wood cells control sap（树液，精力） flow and protect trees when they are injured.

The study, featured on the cover of the September issue of the American Journal of Botany, used a special microscope to reveal how nanostructures help contain damage within microscopic cavities called bordered pits in wood-fiber cells. The pits allow sap to circulate through adjacent（邻近的） cells.

Previously, wood had to be dried, coated with carbon, and put under a high vacuum to be studied at the nanolevel. But using a powerful new type of confocal, laser-scanning microscope, the 4Pi, the scientists discovered that the bordered pits are filled with a mesh membrane of nanofibrils, which radiate from a thickened, solid central region called the torus -- like a bull's-eye in a target.

Fluids ooze in, through the mesh-like membrane, around the torus, and out the other side.

"When wood is injured, such as by an insect, or is being dried, like we do with kiln-dried lumber, the torus and part of the membrane will typically shift from the center of the pit to one side to seal the pit opening," said Barry Goodell, a professor of sustainable biomaterials in the College of Natural Resources and Environment at Virginia Tech. "When we used specific chemical tags to dye the pectin red and cellulose green, we saw for the first time that pectin surrounds the torus and forms ring-like structures at the margin of the torus. Scientists had never seen these before, nor knew there is pectin in these unique ring-like structures in the membrane. As part of the pit-sealing process, pectin separates to form an outer fringe（边缘，刘海）around the torus, while the bulk of the pectin gets pulled into the aperture to block it.

"It helps explain how trees seal off the cells of wood so that softwood trees don't 'bleed to death,' or lose all their sap when they are injured," Goodell said. "Without pectin, a tree cannot defend itself, and it can't seal off damaged cells."

The American Journal of Botany is one of the 10 most influential journals over the last 100 years in the field of biology and medicine, according to the BioMedical & Life Sciences Division of the Special Libraries Association.

The study was written by Daniela Maschek, a student at Georg-August University of Gottingen, who worked in Goodell's lab. Additional collaborators include Jody Jellison, a professor of plant pathology and associate director of the Virginia Agricultural Experiment Station; Mark Lessard of the Jackson Laboratory, and Holger Militz, a professor at Georg-August University of Gottingen.