The present study will help answer questions of tree type evolution, function, optimum, and tree construction types, using the approach of constructional morphology which to date has been widely neglected in palaeobotany and botany. First, the evolution pattern of the earliest Devonian trees is analyzed and explained, including a brief introduction of tree biomechanics. Then fossil and recent trees are studied from the viewpoint of constructional morphology with the main emphasis on the trunk as the most characteristic element of a tree. The various trunk constructions are classified into functional construction types, which are described and analyzed with respect to their biomechanical and biological properties. This functional comparison shows that the basic trunk constructions all appear in the Devonian, have specific advantages and disadvantages and constrain the possible growth habit of a tree. This study based on modern and fossil trees not only leads to a description but also to a causal understanding of the evolution and biology of the various tree types.