Scanning electron micrograph of the bacteria Agrobacterium tumifaciens grown on surface cells of Nicotiana plumbaginifolia, the tobacco plant. These soil bacteria, carry a circular piece of DNA called a Ti-plasmid, which induces tumours (cancer) in plants. Tumorous plants contain several unusual amino acids named opines", and different strains of the bacterium has distinct opines which they use as food. The bacteria are natural genetic engineers, able through the insertion of plasmids, to divert a plant cell into producing food supplies. Ti- plasmids are studied as vectors for the genetic manipulation of plants. Mag: X3600 (at 35mm size)."