Ivy’s Secret Is in Its Secretions

Michael A. Greenwood

As anyone knows who has ever been to Wrigley Field in Chicago, ivy climbs, it covers and it spreads over almost anything in its path.

The plant’s famed surface-climbing ability has made it a fixture in country gardens, on urban buildings and in the ballpark of the Chicago Cubs. But how does ivy inch its way upward? Even Charles Darwin was intrigued by the root-climbing evergreen.

Researchers used an AFM to image nanoparticles deposited on a mica surface. Courtesy of Mingjun Zhang.

Researchers from the University of Tennessee, Knoxville, and from Agilent Technologies in Santa Clara, Calif., have pinned the plant’s talent on the secretion of nanoparticles. The effect is similar to that of glue, binding the plant to materials such as wood, stone or metal and allowing it to scale to ever greater heights.

The findings could be used to synthesize nanoparticles biologically, or they could lead to new approaches for the design and construction of climbing mechanisms for engineering applications, said Mingjun Zhang, a member of the research group from the University of Tennessee.

Using an Olympus optical microscope and an Asylum atomic force microscope (AFM), investigators first studied the morphology of the secretions. Through the microscopes, they saw that the ivy consists of rootlets, at the tip of which are large numbers of adhering discs. These discs, in turn, are broken down into four to seven even smaller fingerlike tendrils. These fingers, measuring about 250 to 350 μm in length, come into direct contact with a surface and secrete the nanoparticles.

To examine the details of the secreted matter, the investigators allowed ivy samples to grow onto silicon wafers and pieces of mica. After a week, the plants were removed, and the nanoparticles’ traces left on the surfaces were inspected with the AFM. They found that the ivy had deposited a highly uniform collection of nanoparticles about 70 nm in diameter and with an average height of 20 to 30 nm.

The investigators further probed the nanoparticles with high-speed liquid chromatography and mass spectrometry to determine their chemical content. Because there were so many components, the investigators focused on 19 compounds that were particularly intense. These are known for their hydrogen bonding, and the composition suggests that the nanoparticles rely on hydrogen bonding for the plant’s clinging ability.

A yellowish gellike substance was observed being secreted from the ivy. When the material dried, the ivy was firmly attached to the surface. Millions of these connection points result in a strong bond between the ivy and whatever it is growing on.

The investigators noted that the nanoparticle secretion system that ivy develops is especially advantageous for surface climbing. With the secreted matter, ivy does not need joints or advanced mechanics to climb, and the plant is highly adaptable to various environments.

Nano Letters, ASAP Edition, March 21, 2008, doi: 10.1021/nl0725704.