Developing aerial roots of common ivy, Hedera helix. The picture shows a stem of an ivy plant that is growing against a wall. The stem is not in contact with the wall, but is producing clustered aerial roots on its shaded side (wall to left). One cluster extends for 2cm along the stem; the other is visible adjacent to a small leaf. The roots are growing away from the light; an example of negative phototropism. Their function is to attach the plant to its support, whether tree bark or the wall of a house. The roots extend until they touch the support, then penetrate any available crevices. Large crevices can accommodate whole roots; small crevices can be penetrated by outgrowths from individual cells. This self-supporting mechanism enables the plant to climb to considerable height. This in turn results in exposure to higher light levels, allowing the production of flowers and seeds