One of the major aims of this book is to appraise critically the role that PGRs play in plant development. Certain developmental phenomena which have been reported to be influenced by PGRs, such as juvenility, vernalization and the photoperiodic induction of flowering, involve the transition of shoot apices from one state of differentiation to another. These observations raise the possibility that PGRs could have a role to play in regulating the differentiation of plant cells per se. The process of differentiation is of fundamental significance to our understanding of plant growth and development, and yet studies in this area have been rare. This chapter examines the role that PGRs may play in influencing apical transitions, and considers the mechanisms which could regulate cellular differentiation and morphogenesis.