Figure 6.

Models for the regulation of phase change. ➞, Primary signals; →, secondary signals. (a) The meristem-autonomous model postulates that vegetative identity is regulated entirely by the SAM. Thus, the primary event in phase change is a change in the identity of cells in the SAM. (b) This diagram illustrates the hypothesis (26) that adult leaf identity is regulated entirely by interactions between preexisting leaves and newly formed leaf primordia. (c) The meristem-patterning model proposes that vegetative identity is regulated by factors that act independently on existing leaf primordia and the SAM. The maintenance of developmental phase is regulated in part by changes in the character of the SAM.