Wood anatomy is known to be untimely correlate with climate; the resumption of cambial activity in spring is brought both by internal chemical factors and external conditions. The understanding of tree-ring development over a short temporal scale is essential in order to understand growth dynamics in trees and the main factors affecting xylogenesis. It is one of the major issues when assessing precise relationship between wood anatomy and meteorological events and thus climate. Although wood and fossil wood is of primary importance to us, the timing and mechanism of wood formation is still poorly known; almost all the studies dealing with homoxylate wood: balsam fir (Abies balsamea), larch (Larix sibirica), scots pine (Pinus sylvestris), Pinus cembra, Picea abies. The observations made during our study could be particularly meaningful for interpreting fossil wood and could bring to us a better knowledge of the window during which the climatic signal directly enter the tree. How do seasons print on the wood? Which part of the wood react to the different meteorological events and how? The objectives of this study were to develop a method of following wood formation, determine intraspecific variation in phenology between sites in different geographic locations whit different precipitation regimes, and compare interspecific patterns in wood formation within sites during one year. We have worked on tree common species, chestnut, Castanea sativa Mill. beech, Fagus sylvatica L, in two french regions. Samples have been taken since February 2005 until December 2005. Cambial reactivation occurs a few weeks after bud flushing. The length of the wood formation period is relatively short, from the end of April for chestnut, May for beech, to August. The two species have a more or less pronounced sigmoid shape in their wood formation, with a slow beginning, followed by a fast growth in the middle of the vegetation period (about June) and a decreasing wood formation activity towards the end of the period (August). But no time-lag seems to happen between the two regions, either in the date of the beginning of wood formation or in the duration of xylogenesis. Climatic factor impacts are now being studied in order to search for markers usable in palaeobotany.