Organogenetic capacity of leaves: the significance of marginal blastozones in angiosperms

A new term — the "blastozone" — is proposed to designate regions of the shoot competent for organogenesis. It is argued that the notion of "marginal meristems" is based on the cell theory and thus may not be appropriate to elucidate the process of organ formation. For instance, with respect to the occurrence of initials and of an elevated cell division rate marginal meristems have been shown to be doubtful structures. Furthermore, organogenetic competent regions form only parts of the meristems of the shoot. The study of blastozones from an organismic perspective reveals primary morphogenetic events such as initiation, incorporation, and fusion processes. Loss of morphogenetic competence is associated with histogenetic events, e.g., trichome outgrowth, and indicates the onset of processes leading to maturation. The marginal blastozone of the leaf is then used up although meristem features continue to be expressed. A series of SEM studies in several genera exemplifies the proposed viewpoint, demonstrating some of the morphogenetic potentialities of angiosperm leaf marginal blastozones.