Gravitropism - knowing up from down: It's not something we often think about, but it's a critical factor in plants and animals. It's similar to plants responding to light, but gravitropism is the response of plants and animals to gravity. It helps us stand straight and it helps plants properly reorient themselves if they tilt because of environmental factors like wind or obstructions. The ability of one fungus species to detect up from down seems to have come from a DNA transfer (and subsequent mutation) from a bacteria. Mitochondria are thought to be the result of bacterial inclusion in cells. How many other features in plants and animals are the result of bacterial DNA transfers?