In general, it can be said that secretion is the accumulation of secondary products resulting from metabolism that are not going to be immediately used and of primary metabolites that are going to participate in metabolic reactions in the cell. Secretory cells arise from epidermal or parenchyma cells and do not form real tissues. The structures responsible for the secretion can be found in both internal and superficial parts of the plant and can be a single secretory cell or multicellular secretory structure.
1. Superficial secretory structures
There are many secretory cells differentiated from epidermal cells on the surface of plants.
Hydathodes are structures that release water and some substances from the interior of the leaf to its surface. This process is called guttation and occurs by water pressure coming from the root. Hydathodes are modifications of the leaf found in the leaf margins or at the tip.
Nectaries are secretory structures that release sugar-rich solutions made from the substances supplied by the phloem. Two categories can be distinguished: floral nectaries, which are directly associated with pollination, and extrafloral nectaries, which are found on the vegetative parts of the plant. There is a broad diversity of nectary structure, from simple epidermal glandular surfaces to more complex structures.
Osmophores are secretory structures that produce the fragrance of flowers through the secretion of volatile substances. Plants growing in saline habitats (halophytes) have salt glands to secrete ions for regulating the salt content of plant shoots.
Glandular trichomes are superficial secretory structures. For instance, the fragance of plants is mostly because of the volatile substances released by these structures. Glandular trichomes are commonly multicellular, with glandular cells at the distal end attached to the epidermis by non-glandular cells known as basal cells and by others forming a stalk or peduncle. The secreted compounds may remain on the epidermal surface or be released into the environment. Glandular trichomes are classified according to the substances they release or their morphology. They can show one or a few secretory cells.
2. Internal secretory structures
Internal secretory compounds are products stored inside plant tissues, sometimes throughout the lifespan of the plant. Internal secretory structures are far from the epidermis and are located mainly in the cortical parenchyma of stems, leaves, roots, and fruits. We can distinguish three types of internal secretory structures:
a) Secretory cells. They synthesize and store a wide variety of products, such as resins, mucilage, tannins, oils, gums, and even crystallized substances. They are highly specialized cells often referred to as idioblasts.
b) Secretory cavities and ducts (canals) differ from secretory cells in that they secrete substances into intercellular spaces. These spaces can be formed by cellular separation (schizogeny), as is the case of most resin ducts. They can also be originated by the disintegration and dissolution of the glandular cells (lysogeny), like the lysogenic cavities of the citric fruits.
c) Laticifers refer to individual cells or groups of connected cells containing a liquid called latex. Laticifers are found in a wide variety of species, from herbaceous to woody plants.
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Bibliography ↷
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Dassanayake M, Larkin JK. 2017. Making plants break a sweat: the structure, function, and evolution of plant salt glands. Frontiers in plant science. 8: 406.
Lange BM. 2015. The evolution of plant secretory structures and emergence of terpenoid chemical diversity. Annual review of plant biology. 66: 139-159. doi: 10.1146/annurev-arplant-043014-114639.
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