Secretory Ducts and Laticifers
In many plants, substances such as oils, resins and mucilage are secreted internally, often into specialized ducts formed either by cell wall separation (schizogenous ducts) or cell wall degradation (lysigenous ducts), or a combination of the two processes (schizo-lysigenous ducts). Some angiosperms, especially eudicots such as Euphorbia and Ficus, produce latex from specialized cells (latici-fers) that permeate their tissues (Figs 1.4, 1.6). In Euphorbia, the laticifers are derived from a small group of initial cells in the cotyledonary node of the embryo; these cells are coenocytes, since they undergo repeated nuclear divisions without corresponding wall formation. They grow intrusively between cells of surround-ing tissues, and often branch and eventually ramify throughout the entire plant. Coenocytic laticifers are termed non-articulated laticifers. By contrast, laticifers of a few species (e.g. Hevea brasiliensis, the source of commercial rubber) undergo cell-wall formation, and thus consist of linked chains of cells; these are termed articulated laticifers. Laticifers of the opium poppy (Papaver somniferum) are always associated with vascular bundles; the alkaloids produced in the latex of these cells are the source of narcotic analgesics such as morphine.