A young growing shoot of Hydrangea petiolaris, showing the development of adventitious aerial roots on one side of the stem. H. petiolaris is a woody climber; it uses aerial roots in order to attach itself to the bark of trees, to rocks, or, in gardens, to walls. The roots grow in closely packed rows, and emerge on the side of the stem away from the light. In this picture, the light was to the left, and a house wall to the right. Underground roots of plants show geotropism; they grow downwards. In the case the of aerial roots shown here, there is no geotropic response, and grow is directed towards low light levels. When the roots eventually touch a surface, the tip attaches itself, and later the root becomes covered with a lignified cortex, which strengthens the bond between the plant and its substrate. The roots in this picture are up to 7mm long