Conceptual Overview

While secretory activities are found in virtually all plant cells at some time during their life, there are individual cells (idioblasts) and cell complexes whose primary function is to perform secretion on a regular basis.

Secretory cells may be classified by their location in the plant, or on the basis of the product that is exuded. There are external and internal secretory systems identified on the basis of their position. Examples of external are glandular trichomes (glandular hairs), or glands of various morphology mainly of epidermal origin. They are formed on the surface of the above ground parts of plants, such as stems, leaves, bud scales and reproductive organs. Internal examples include secretory cavities and canals (ducts) which are intercellular spaces lined with epithelium cells as well as idioblasts. They are found in tissues such as xylem, phloem, cortical and pith parenchyma. In regard to the secreted materials, there are nectaries, hydathodes, salt glands, crystal-containing idioblasts, tannin cells, ethereal oil glands and cells, resin ducts, colleters, protein, slime, and gum-secreting systems.

Nectaries secrete a sugar solution (nectar). They are most often found on flower parts (floral nectaries), but may also develop on leaves and stems (extrafloral nectaries). Floral nectaries may be located in many floral parts such as stamens (intrastaminal nectary) or at the base or on top of the ovary (septal nectaries). Floral nectar attracts insect pollinators of entomophilous plants while extrafloral nectar may attract ants which defend the plant from herbivorous insects. Nectaries vary greatly in morphology and anatomy. Extrafloral nectaries are usually represented by glandular hairs or glandular epidermis. Floral nectaries usually form multicellular outgrowths on the flower parts. The secretory (glandular) tissue of these nectaries consists of an epidermis with stomata and with or without glandular trichomes and specialized parenchyma of small densely cytoplasmic cells often called nectariferous tissue. Nectar is exuded through stomatal pores and (if present) glandular trichomes.

Hydathodes are external secretory structures exuding water which appears as droplets on the surface of the organ in a process called guttation. Two types of hydathodes are recognized: 1 active: usually in the form of glandular trichomes in which water is actively exuded by secretory cells which are not connected to water-conducting tracheary elements, and 2 passive: usually located at leaf margins or tips of leaves. In passive hydathodes, water is released from the tracheary elements and then passes through intercellular spaces of the epithem, or modified mesophyll cells. Eventually, water is exuded onto the leaf surface through modified stomata which are permanently open. Cells of the epithem have little or no chlorophyll and, when viewed with transmission electron microscopy, show primary wall structure characteristic of transfer cells.

Ethereal oil glands, colleters and resin ducts produce mainly terpenes, i.e. lipophilic substances that provide defense against insects and fungi.

Secretory products may be released from the cell into canals (as in resin ducts), subcuticular cavities (as in glandular trichomes), or onto the surface of the organ (as in nectaries, hydathodes and salt glands). They may also accumulate in a vacuole of a cell in the form of crystals (as in crystal-containing idioblasts) or amorphous inclusions (as in tannin and oil cells). In the latter case it is often spoken as of internal (intracellular) secretion.

A special type of internal secretory system is composed of laticifers that produce milky substances, primarily latex. Laticifers are either chains of fused cells whose cross-walls are digested in the center, or single cells with nearly unlimited growth. The first are termed articulated laticifers (or laticiferous vessels); the second ones non-articulated laticifers. Non-articulated laticifers grow intrusively (pushing aside neighboring cells, they undergo mitoses which are not accompanied by cytokinesis). As a result, a multinuclear (coenocytic) cell is formed. Both articulated and non-articulated laticifers may be branched (i.e. anastomosis). Latex is a milky fluid that is commonly composed of both organic (alkaloids, rubber particles) and inorganic substances in an aqueous phase.

Another special type of the external secretory structures are stinging hairs producing toxic substances which are stored in cell vacuoles.