The late Devonian-early Carboniferous period sees the replacement of forests dominated by the first large woody trees (Archaeopteris) by more diverse forests with new types of trees. However, the scarcity of deposits with well-preserved plants from this period hampers our understanding of the processes underlying this major transition.Here, we propose to apply synchrotron technology to image the internal structure of selected plant fossils from latest Devonian outcrops of Ireland recently collected by our team. The fossil plant tissues are extremely well-preserved in 3 dimensions, but their study using classical methods has proven difficult. The objective is to reconstruct the 3D architecture of several minute fossil branches and roots and the characteristics of their tissues and cells. This will provide new data on the anatomy and, by extension, on the physiology of plants growing in the paleotropics about 360 million year ago, right before the Devonian-Carboniferous boundary.