This image from the Mars Reconnaissance Orbiter appears to show some type of vegetation growing on Mars. Many have suggested that they resemble Martian trees. However, this is really only an optical illusion caused by a trick of light and shadow. The dark streaks are actually dark sand that has been pushed out of the dunes by carbon dioxide gas as it melts beneath the surface. The dark streaks run down the surface of the dunes creating this uncanny illusion.
NASA / JPL / University of Arizona