Procambium–cambium transition during vascular meristem development in Diospyros lotus Elz ˙ bieta Mys ´ kow Abstract: Trees owe their growth in girth to cambial activity that gives rise to secondary conducting tissues. Since cambium originates from the primary vascular meristem, called procambium, the main objective of the present study was to analyse the changes occurring during transition from procambium to cambium in Diospyros lotus L., characterized by the occurrence of double-storied cambium. In this species, procambium and primary vascular tissues are present only within the bud, whereas just below this level, cambium and secondary vascular tissues occur. Procambium differentiates as a continuous cylinder, which as a whole transforms into cambium during further development. Commonly used criteria distinguishing procambium and cambium — that is, the appearance of periclinal divisions, intrusive growth of meriste- matic cells, presence of rays, and primary versus secondary nature of derived vascular tissues — were analysed in D. lotus. All these features, however, were present in procambium, making the sharp distinction between both stages of vascular meristem based on these criteria difficult. In addition, the formation of fusiform and ray initial storeys in D. lotus was shown to appear in the third year of cambium activity and become more distinct with cambium age. The double- storied structure may result from the genetic control of cell fate and (or) the presence of epigenetic patterns determining cellular arrangement. Key words: procambial and cambial initials, periclinal divisions, primary rays, tracheary elements, vascular system. Re ´sume ´: La croissance des arbres en diame `tre provient d’une activite ´ cambiale donnant naissance aux tissus conducteurs secondaires. Sachant que le cambium tire ses origines du me ´riste `me vasculaire primaire, ou procambium, l’auteur s’est propose ´ d’analyser les modifications survenant au cours de la transition du procambium au cambium chez le Diospyros lo- tus L., caracte ´rise ´ par la pre ´sence d’un cambium a ` deux e ´tages. Chez cette espe `ce, l’on ne retrouve le cambium et les tis- sus vasculaires primaires que dans le bourgeon, alors que l’on retrouve imme ´diatement sous cet e ´tage, le cambium et les tissus vasculaires secondaires. Le procambium se diffe ´rencie comme un cylindre continu se transformant dans l’ensemble en cambium au cours du de ´veloppement ulte ´rieur. Les auteurs ont analyse ´, chez le D. lotus, les crite `res usuels distinguant le procambium et le cambium, c.-a `-d. apparence des divisions pe ´riclinales, croissance intrusive des cellules me ´riste ´mati- ques, pre ´sence de rayons, et nature primaire vs. secondaire des tissus vasculaires de ´rive ´s. Cependant, l’ensemble de ces ca- racte `res se retrouve de ´ja ` dans le procambium, rendant ainsi difficile la distinction nette entre les deux stades du me ´riste `me vasculaire, base ´e sur ces caracte `res. De plus, on de ´montre que la formation des couches de cellules initiales des cellules fusiformes et des rayons chez le D. lotus n’apparaı ˆt qu’apre `s trois ans d’activite ´ cambiale, tout en devenant plus distinctes avec l’accroissement en a ˆge du cambium. La structure en deux e ´tages pourrait re ´sulter d’un contro ˆle ge ´ne ´tique du sort des cellules et (ou) de la pre ´sence de patrons e ´pige ´ne ´tiques de ´terminant l’arrangement cellulaire. Mots-cle ´s : initiales procambiales et cambiales, divisions pe ´riclinales, rayons primaires, e ´le ´ments de trache ´es, syste `me vas- culaire. [Traduit par la Re ´daction] Introduction Trees have developed a set of anatomical and physiologi- cal features in the course of evolution, which have adapted them to a long life cycle. Specifically, the presence of sec- ondary conducting tissues determines the ability to transport water and nutrients over long distances and increases their mechanical stability. Additionally, extensive development of secondary xylem makes it possible for gigantic trees such as Sequoia sempervirens, Sequoiadendron giganteum, and Eu- calyptus regnans to develop (Lachaud et al. 1999). Produc- tion of secondary tissues results from the activity of a lateral meristem, cambium. Cambium is defined as a zone contain- ing undifferentiated cells located between secondary xylem and secondary phloem, that is, initial cells together with de- rivatives that form xylem and phloem (Romberger et al. 1993). In stems of long-lived plants, cambium usually origi- nates from procambial cells located between primary phloem and primary xylem within vascular bundles (fascic- ular cambium) and from parenchyma cells that separate vas- cular bundles (interfascicular cambium). In such stems, interfascicular parenchyma dedifferentiates to form the con- Received 20 August 2010. Accepted 30 September 2010. Published on the NRC Research Press Web site at botany.nrc.ca on 28 October 2010. E. Mys ´kow. Institute of Plant Biology, University of Wroclaw, Kanonia 6/8; 50-328 Wroclaw, Poland (e-mail: myskow@biol.uni.wroc.pl). 985 Botany 88: 985–993 (2010) doi:10.1139/B10-070 Published by NRC Research Press