Plant Physiology - Musienko, M. M. 2001

Physiology of Excretion in Plants
Internal Secretory Structures

They differ little from the basic parenchyma, yet they contain various resins, oils, Tannins, mucilages, and even crystals. Such Cells are also referred to as secretory idioblasts. In addition, secretory cavities of various shapes are formed: rounded (Myrtaceae) or elongated into channels (Compositae). Their exudates may include volatile Terpenes, balsams, gum-resins, and latexes. In some plants, resinous substances are secreted into intercellular spaces, where they form a granular layer along The Cell walls.

In multicellular structures, most terpenoids are exported from the cell, whereas in idioblasts and laticifers (cells or rows of cells containing latex — milky sap), they accumulate within the vacuoles. Laticifers vary widely in their Structure, just as latex varies in composition.

More than 12,500 species of latex-producing plants are known, ranging from herbaceous spurges (Euphorbia) to rubber-yielding trees (Hevea).

A distinction is made between solitary (non-articulated) laticifers and fused cells (articulated laticifers). Similar to cell sap, latex is a suspension containing CARBOHYDRATES, organic acids, Alkaloids, sterols, fats, tannins, etc. Secretory cells are characterized by an agranular Endoplasmic reticulum, which is likely involved in the synthesis of terpenoids. In laticifers, the protoplast is living, the Cytoplasm is parietal, a Nucleus is present, and the entire remaining volume is occupied by latex. These cells are typically characterized by the presence of leucoplasts, Mitochondria, and lipid droplets. Interestingly, the walls of laticifers do not undergo lignification; they are apparently pliable. Callose has also been detected in laticifers.

Most experimental data indicate that latex—specifically essential oil and resin components—is synthesized in the agranular ER, mitochondria, Plastids, and even The Nucleus. Via eccrine secretion through the Plasmalemma, they enter the free space of the cell walls, and from there into the channels of resin ducts or to the outside of the plant. Most of these products are derivatives of isoprene (C₅H₈). Laticifers exude latex when ruptured. Undamaged vessels remain turgescent, i.e., in a state of osmotic equilibrium with the surrounding parenchyma. Because latex readily absorbs Water from neighboring cells, it is believed to be involved in the Regulation of Water balance. Various hypotheses have been proposed regarding their potential function, but the prevailing view is that laticifers function as the plant's excretory system.

At the same time, the wide variety of substances found in latex (linalool — lily of the valley, coriander; geraniol — rose oil, geranium and eucalyptus oils; limonene — dill and caraway oils, turpentine; menthol — peppermint; rubber and gutta-percha, among many others) suggests that laticifers may perform a broad spectrum of diverse Functions.