Crown gall on the trunk of a wild cherry, Prunus avium. The picture shows a tumorous outgrowth resulting from infection by the bacterium Agrobacterium tumefaciens. Galls of this nature are widespread on a variety of both woody and herbaceous plants; the infection usually begins with the bacterium entering surface wounds. Treatment is rarely necessary and largely ineffective. The importance of A. tumefaciens lies in the mechanism of tumour initiation. The bacterium contains small circular pieces of DNA - the Ti plasmid - containing about 100 genes. Some become integrated into the host DNA; once infected, the gall can grow in the absence of the bacterium. This method of altering plant DNA has become the basis of modern recombinant DNA technology. Plasmids can be modified in vitro, and used as a vector to introduce new characteristics into a host; eg herbicide resistance in crop plants