The maturation process is a general phenomenon which affects arborescent species according to their ontogenetic development from seed. One of its major consequences is the decrease, or even the Loss, of ability for cloning as trees reach an age corresponding to a suitable size for a reliable evaluation of their potential. It is clear that shoot apical meristems, through their morphogenetic functions, play a key role in ontogenetical ageing. Nevertheless, current alvances also suggest a fundamental importance for the physiological aspects of phase change and their connected space-time variations. A number of studies suggest that a juvenile phase, with maximal intensity corresponding to the onset of each shoot flush, exists in apical meristems, this particular stage becoming more and more space-time reduced as tree architecture increases in complexity. Understanding of this reiterative pattern of meristem phase change should enhance prospects for successfully cloning mature trees. New developments in biochemical investigations offer promise of further progress in maturation process studies from a basic as well as a practical point of view.