The woody parts of a tree can be divided into aerial xylem and root xylem. Trunk and branch xylem in a given species usually have similar cell distribution patterns whereas roots are often anatomically different. In pedunculate oak (<i>Quercus robur</i> L.) the aerial xylem is ring porous and the roots are either radial porous or diffuse porous. The roots of all species of oak usually have xylem with indistinct growth ring boundaries. In contrast, in European beech (<i>Fagus sylvatica</i> L.) the xylem in all parts of the tree is diffuse porous or semiring porous and has well defined growth ring boundaries. The differences between aerial and root xylem in oak and beech are shown with particular reference to how well the growth rings are defined, and two other British tree species with ring porous aerial xylem, European ash (<i>Fraxinus excelsior</i> L.) and English elm (<ilmus procera</i> Salisb.) are illustrated for comparison. The poorly defined growth rings in many root samples of these and other species (particularly those with ring porous aerial xylem) will often result in difficulties with their dating and aging.