Vascular plants are characterized by a primary growth in which organs are produced by apical meristems and by a secondary growth in which lateral meristems increase the girth of organs. While the molecular events associated with the differentiation of primary and secondary tissues begin to be unravelled, little is known about the transition between these two developmental stages which is a progressive and continuous process. The perennial life of trees is ensured by the regular production of new xylem and phloem tissues by the vascular cambium. The cambium is made of initials which divide inwards in xylem mother cells and outwards in phloem mother cells. Two types of initials coexist within the cambium: the axillary elongated initials at the origin of the axial system and the smaller isodiametrical ray initials responsible for the formation of the radially orientated parenchymatous rays. The rays form a connection between phloem and xylem that allows the translocation of nutrients and the transmission of signals both axially and radially. In this issue, the molecular changes associated with the setting up of secondary growth in Populus tremula×P. alba are addressed by van Raemdonck et al. The cover was designed by Pierre Martens. (See van Raemdonck et al., pp. 2211–2227.)
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