Aerial Roots of Ivy, Hedera helix

Aerial roots of common ivy, Hedera helix. The picture shows an ivy stem on the branch of an apple tree (grey, background). The stem is several years old and is attached to the tree by means of aerial roots (brown filaments). Those roots in intimate contact with the tree bark (to left of stem) are pale in colour and retain their turgidity. They are closely adpressed to the tree, and this clinging bond, which supports the ivy stem, also protects them from dehydration. By contrast, the roots which have developed away from the bark (to front, e.g top right) are darker in colour and - exposed to air on all sides - have shrivelled. Aerial roots are not sufficient to support the life of the ivy plant; if the stem is cut, the foliage above the cut dies. Ivy is not a parasite, but a valuable species for wildlife, providing food and shelter. A heavily laden tree has a higher risk of windthrow., by DR JEREMY BURGESS/SCIENCE PHOTO LIBRARY