Atlas of plant and animal histology

Resin ducts are internal secretory structures mostly developed in a schizogenic way. They are typical of conifers, where they can be found in ground tissues (among cortical parenchyma cells, in both leaves and stems), as well as in vascular tissues (among secondary xylem cells of stems). They consist of elongated structures mostly originated by the separation of cell walls of adjacent cells resulting in a cavity or duct lined with an uniseriate (as in the xylem) or biseriate (in the cortical parenchyma) epithelium of resin-secreting cells (Figures A and B). In Pinus, the epithelial cells are generally thin-walled, remain active for several years and produce abundant resin; the resin is a mixture of substances composed mainly of terpenes. The toxic resinous compounds protect the plant from a wide range of herbivores, insects (such as the beetles), and pathogens (such as fungi); while the volatile phenolic compounds may attract benefactors such as parasites or predators of the herbivores that attack the plant.