The shedding of branches along a preformed layer is a complex phenomenon in which several types of xylem elements are involved. The abscission layers in Perebea mollis and Naucleopsis guianensis are distinguished from the normally developed xylem by the presence of shorter unlignified fibres with small simple pits. Most of these fibres show large nuclei. In this zone containing abnormal fibres, the vessels, rays and parenchyma are conspicuously different from those of the normal tissue. The transition in anatomy from the main branch to the abscission layer is much more abrupt than that from the abscission layer to the secondary branch. Contrary to what is generally seen in other Moraceous tribes, natural pruning seems to occur in all genera of Olmedieae with the sole exception of the genus Olmedia. The correlation with other morphological and anatomical characters and the complexity of the phenomenon of branch abscission seem to justify Berg’s decision to use this character in defining the new tribe Castilleae, containing all the genera formerly placed in Olmedieae except Olmedia.