In plants, organogenesis, which is simply the process of forming new organs, occurs continously and only stops when the plant dies. In the shoot, the shoot apical meristems regularly produce new lateral organs (leaves or flowers) and lateral branches. In the root, new lateral roots form from weakly differentiated internal tissue (e.g. the xylem-pole pericycle in the model plant Arabidopsis thaliana). In vitro and in response to specific cocktails of hormones (mainly auxins and cytokinins), most plant tissues can de-differentiate and form a mass of dividing totipotent stem cells called a callus. De novo organogenesis can then occur from those cells. The type of organ that is formed depends on the relative concentrations of the hormones in the medium.