DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY
Developing aerial roots of common ivy (Hedera helix). At right is a piece of ivy stem from a plant growing against a wall. The comb-like aerial roots (cream, horizontal) are emerging in the direction of the wall (not visible, but to the left). The roots grow away from the light; an example of negative phototropism. Their function is attach the plant to its support, whether the bark of a tree, or the wall of a house. The roots extend until they reach the support, where they penetrate any available crevices. Large crevices can accommodate whole roots; small crevices can be penetrated by outgrowths from individual cells. The roots in this picture are 1-5 millimetres long, and had not reached the wall. Ivy is often regarded as a danger to masonry work; however, its presence on a building may conversely be beneficial due to its insulating and shading effects.
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