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In plants, it is difficult to distinguished between excretion and secretion processes, since the products of both of them are usually stored in the same plant compartments. In general, it can be said that the secretion is the accumulation of secondary products resulting from metabolism that are not going to be immediately used and primary metabolites that are going to immediately participated in metabolic reactions in the cell. Secretory cells arise after the differentiation of epidermal or parenchyma cells, and do not form real tissues. The structures responsible for the secretion have highly diverse morphologies and are located in diferent parts of the plant. In fact, they can be found in both internal and external parts of the plant, and can be a single secretory cell or multicellular secretory structure. In addition, they can produce a wide variety of products. Here, we are going to classify secretory structures according to their location inthe body of the plant: external (the surface) or internal.

External secretory structures

There are many secretory cells on the surface of plants, either as unicellular or multicellular glandular trichomes in the epidermis or as part of the epidermal layer. These secretory cells originate by division and differentiation of epidermal cells. Some of them release mainly hydrophilic (water-loving) or lipophilic (water-hating) substances, and others release both hydrophilic and lipophilic substances. There are superficial secretory cells in stems, leaves, fruits and flowers. For example, there is abundant secretion in petals, where the released molecules cause the fragrance of flowers. Although these cells are highly specialized in secretion, all epidermal cells are actually secretory since all of them release substances that are deposited on their walls or released to the environment.

Hydathodes are structures that release water with 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 some parts of the leaves and are located along the margins or at the tip of the leaf. Structurally, hydathodes consist of a) terminal tracheids of leaf nerves, b) the epithem, which consists of thin-walled and few chloroplasts parenchyma cells located at the leaf nerve endings, c) a sheath or envelope that is continuous with the epidermis (the sheath cells may be suberized and may even have Caspary bands), d) an watery opening or pore (watery pores are small nonfunctional stomata who have lost the ability to regulate opening and closing). Although this is the general organization, there may be variations, such as lack the sheath, or the epithem, or may even have an opening which is not a stoma.

Although hydathodes are normally found at the margins and tips of leaves, in some species, they also appear on the surface and are called laminar hydathodes. Hydathodes are generally associated with the release of water from the plant tissues, but in many xerophytic species they are specialized in the absorption of condensed fog or dew water, i.e., the opposite process. Some authors have described the active hydathodes, also known as trichome-hydathodes, as glandular trichomes that release water. These structures would release water without being influenced by the osmotic pressure, hence they are considered active as opposed to the others that would be passive.

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 huge variation in nectary structure, from simple epidermal glandular surfaces to more complex structures.

Osmophores are secretory structures that produce the fragrance of flowers through secretion of volatile substances. Plants growing in saline habitats (halophytes) have salt glands to secrete ions, which is the best known mechanism for regulating the salt content of plant shoots.

Glandular trichomes are external secretory structures, which are present in about 30% of plant species. For instance, the smell 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 other forming a stalk or peduncle. However, some thrichomes are unicellular. Glandular cells have a primary cell wall, sometimes covered by a cuticle at the joining area with the stalk cells, and contain a cytoplasm with abundant organelles such as mitochondria, endoplasmic reticulum and Golgi cistern stacks. However, there are differences between glandular cells of different species. For example, glandular cells releasing terpenes contain very scarce Golgi apparatus or it is not visible at all.

Glandular trichomes
Trichomes of a mallow leave.

Glandular trichomes contain cells that release substances with different functions to the environment, either volatile or substances that remain on the surface of the plant. Glandular cells can synthesize the molecules to be released, even they have sometimes the ability to perform photosynthesis. It means that some glandular cells may live as rather independent units. However, in other cases the molecular building blocks for synthesizing the compounds to be release are coming from the underlying plant tissues through the stalk cells of the trichome.

Glandular trichomes are classified according to the compounds they release or the trichome morphology. There are thichomes with a basal cell, one or more in the stalk, and a few secretory cells at the distal end. These trichomes commonly release not very volatile substances that are laid on the surface of the plant. Other types of trichomes show a basal cell, a short cell in the stalk, and a head with one or several secretory cells showing a large cavity between the cuticle and the primary cell wall, which is filled with substances. There are many other examples of glandular trichomes. It can be found different types of trichomes in the same plant, even in the same organ. For example, in carnivorous plants, there are trichomes for fetching the insect by releasing mucilaginous substances or nectar, and others release proteolitic enzymes for digestion. The amount of trichomes, both glandular and not glandular, is influenced by the environment. For instance, it can be increased after the attack of a herbivore.

The release of secretory compounds may be achieved by different ways. Those secretory products stored between the cell wall and the cuticle are released when the trichome is touched and the cuticle is detached. In other cases, the cuticle has small channels to guide substances to the environment. Some glandular cell store the substances in their cytoplasms so that they are released when the cell breaks. Initially, the precursor molecules arrive to secretory cells through the stalk cells of the trichome, but the final secretory products synthesized in the secretory cells do not travel back to the plant organ. This is because the mature secretory cells get a thicker cuticle and the cells of the stalk cells close to the secretory cells work now as a barrier.

There is a wide variety of glandular trichomes regarding the compounds they release. Some synthesize and release organic acids, others release salt, mucilage (polysacharides), terpens, mixings of terpens and mucilage, nectar, proteolytic enzymes by trichomes of carnivore plant, etcetera. Some of these substances may be antimicrobianes. One important function of glandular trichomes is mediating interactions between plants and animals. For instance, released terpens may attract insects or be toxic to them, produce alarm, regulate insect development, or other effects depending on the insect species. More curious, some terpens attract insect predators or insect parasites. This last strategy is known as indirect defense.

Internal secretory structures:

Internal secretions 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:

Secreting cells. They are isolated cells that differ from the surrounding cells and may be from isodiametric forms to forms enlarged in length like sacs or tubes. They synthesize and store a wide variety of products, such as resins, mucilage, tannins, oils, gums, and even crystallized substances. Many secretory cells contain mixtures of substances, but in many others the contents have not yet been identified. When the secretory cells appear as specialized cells, they are often referred to as idioblasts, more specifically secretory idioblasts.

Conductos resiníferos
Resin ducts.

Secretory cavities and ducts (canals) differ from secretory cells in that they secret substances into intercellular spaces. Secretory cavities are short secretory spaces, whereas secretory ducts are long secretory spaces. These spaces may be located anywhere in the plant and can be formed by two different cellular processes: schizogeny and lysogeny. They are commonly produced by the separation of cells of vascular tissues or ground tissues, resulting in an intercellular space lined with secretory epithelial cells. These spaces are called schizogenous secretory cavities and ducts, as it is the case of most resin ducts. They can also be originated by the disintegration and dissolution (autolysis) of the glandular cells. In this case, the secretory product is formed in the cells that eventually break down, but the compounds remain in the resultant space. These spaces are known as lysogenic spaces, like the lysogenic cavities of the citric fruits. Some authors also recognize a third pattern of development, the schizolysogeny. The formation of schizolysogenous cavities and ducts is initially schizogenous (separation of intact cells), but followed by lysogeny at later stages, when the epithelial cells lining the space undergo autolysis, further enlarging the space. The resin ducts in the bud scales of Pinus pinaster is an example of ducts following a schizolysogenous pattern of development.

Laticifers refer to individual cells or groups of connected cells containing a liquid called latex. When formed of several cells, they can produce complex tube-like structures. According to the structure, they can be classified into articulated and nonarticulated laticifers. Articulated laticifers are composed of longitudinal chains of connected cells, whereas the nonarticulated laticifers consist of long tube-like single cells. Laticifers are found in a wide variety of species, from herbaceous to woody plants. The term latex refers to a heterogeneous secretory substance which greatly varies in appearance (color) and composition (it may contain substances such as carbohydrates, organic acids, salts, sterols, fats, and mucilages), thus resulting in many types of latex.

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