Morphogens are involved in the establishment of positional information that is essential for pattern formation. In plants, the phytohormone auxin displays some characteristics of a morphogen. Gradients of auxin distribution are required for tissue patterning within the embryo and the root apex. In some other instances, such as de novo organogenesis, auxin action can be better described in terms of a morphogenetic trigger, which is defined as a factor that induces, through local increase of its concentration, acquisition of a new developmental fate in plant cells that were originally similar to their neighbours. A morphogenetic trigger specifies the site where a new organ will be formed. In plants, formation of reiterative and modular structures might need the action of both morphogenetic triggers and morphogens.