The plant fossil record preserves a number of morphological and anatomical characters that, combined with our knowledge of extant plant biology, can be studied using structure-function relationships. This approach allows us to analyze the dynamics of plant diversity through a functional prism that complements classical taxonomic approaches. It makes it possible to understand fossil plants as organisms that were once alive and interacted with their environment. This approach is relevant for whole-plants but also to answer functional questions at the level of a single plant organ (e.g., stems, leaves, roots) or tissue (e.g., wood). Here, I will present results obtained on Devonian-Permian fossil trees in the last 10 years. Some traits that are important to understand the biology of the plant are observable directly on the fossils. For example, the presence of a thick bark or of epicormic shoots that can repair the crown of the plant after a disturbance can indicate an ability to withstand stressful conditions. Interactions between plants and organisms such as fungi or insects can also be observed directly, in some cases in great detail. More complex functional traits can also be investigated using a combination of observation and measurements on the fossils and models based on extant plants. I will present three examples of such studies focusing on (1) the biomechanical properties of unusual Permian roots, (2) the growth and amount of carbon stored in one of the oldest trees, and (3) the hydraulic properties of Paleozoic woods.