PLANT EXCRETION: TYPES OF SUBSTANCES, PROCESSES AND STRUCTURES - BIOLOGY - 2025

The plant excretion as such does not exist, because plants do not have specialized structures for this function. An excretion is a physiological process, through which an organism can expel unusable substances or that are toxic to it.

In plants, the excretion function makes it possible to exclude substances that can later be reused in various physiological processes, such as CO ₂ and H ₂ O in photosynthesis and respiration processes, and the accumulation of salts or nutrients in the vacuoles.

Stomata Source: flickr.com

Like all living organisms, plants have a metabolic activity that generates waste products. However, in plants this activity takes place to a lesser degree, since the waste substances tend to be recycled.

The excretion process is carried out by the tissues located along the surface of the plant, mainly in the stem and the foliar area, through the stomata, lenticels and specialized glands.

Various substances produced by plant excretion are very useful for man. Chewing gum, latex or natural rubber, and turpentine are elements that through industrial processes favor human activities.

Types of excretory substances

Depending on your physical state, excretory substances can be solid, liquid and gaseous:

• Solid: such as calcium oxalate salts excreted by the salt glands of mangroves.
• Liquids: such as essential oils, resins, tannins or latex (rubber).
• Soft drinks: such as carbon dioxide product of respiration, and ethylene that contributes to the ripening of fruits.

Depending on their nature and composition, the excretory substances produced by the different metabolic processes are mainly divided into primary metabolites and secondary metabolites.

Primary metabolites

They are the result of primordial metabolic processes such as photosynthesis, respiration, and protein synthesis. Generally these elements, such as water, carbon dioxide or oxygen, are reused in the processes of photosynthesis or cellular respiration respectively.

Secondary metabolites

They are compounds that do not act directly on the essential physiological processes, but contribute to the ecological and adaptation processes of plants.

Terpenoid, alkaloid and phenolic elements are the result of excretion processes of plants with a high industrial, agricultural and medicinal value.

Process

In plants the catabolic rate is low, so metabolic waste is stored slowly, and most of it is reused. Water, carbon dioxide and nitrogenous elements are recycled, reducing the need for excretion.

The excretion process is based on the elimination of waste substances formed in catabolism, osmoregulation and ionoregulation. Plants do not have particular excretory organs, so the substances are discarded through stomata, lenticels or vacuoles.

Structures Involved

Plants lack an excretory system through which to eliminate waste substances. However, it has specialized structures that allow you to delete or store these types of items.

Stomata

Stomata are a group of specialized cells, whose function is to regulate gas exchange and perspiration. In fact, they are located on the surface of the epidermis, mainly in the bundle and the underside of the leaves.

These structures allow the elimination of excess water and gases accumulated inside the plants. During the transpiration process, the plant eliminates water through the stomata, in addition they activate the absorption of liquids.

Perspiration and absorption allow maintaining the osmotic balance within the plant. When transpiration occurs, the plant, depending on the availability of water in the soil, stimulates the absorption of new molecules through the roots.

During the photosynthetic process and respiration, oxygen and carbon dioxide are produced and excreted by plants. The excretion of these elements occurs through the stomata during gas exchange.

Changes in the levels of oxygen or carbon dioxide within the plant stimulate the opening or closing of the stomatal cells. This process is governed by the physiological needs and the environmental conditions in which the plant is located.

Lenticels

Lenticels are structures located on the stems, branches and trunks of woody plants. It consists of an accumulation of loose cells of less suberification that cross the epidermis and communicate the internal cells of the parenchyma with the exterior.

Lenticels. Source: wikipedia.org

Its main function is the exchange of gases from the interior of the plant to the surrounding atmosphere. This is how it intervenes in internal balance, eliminating excess oxygen and carbon dioxide that accumulates in the plant tissues.

Vacuoles

Vacuoles are characteristic cytoplasmic organelles of plant cells, formed by a storage space surrounded by a plasma membrane. They serve to store waste or reserve substances, such as water, sugars, salts, enzymes, proteins, nutrients and pigments.

These organelles allow the cells to keep hydrated, since the vacuolar content influences the increase in turgor pressure. Likewise, they intervene in the disintegration of some substances, recycling their elements within the cell.

Secretory cells

They are specialized cells of parenchymal or epidermal origin, which secrete different substances such as oils, resins, gums, balsams and salts. Examples of these specialized cells are oil cells, mucilaginous cells, and taniferous cells.

Oil cells

Secretion cells at the level of the cortex that contain essential oils. Examples are the aroma of cinnamon (Cinnamomum zeylanicum) that comes off the bark of the plant, or ginger (Zingiber officinale) that has these cells in the rhizome.

Mucilaginous cells

Mucilage storage and secretion cells, a viscous plant substance with a high content of polysaccharides and water. Mucilage accumulates between the cell wall and the cuticle, and is removed when the cuticular tissue is torn.

Taniferous cells

Taniferous cells accumulate tannins that function as defense mechanisms in woody plants against attacks by pathogens and parasites. Tannins are phenolic elements present in plants and fruits, of a water-soluble character, with a harsh and bitter taste.

Specialized glands

Salt glands

The salt glands are vesicular structures located mainly on the surfaces of the leaves. Indeed, they are covered by a cuticle that has tiny pores that connect them with the mesophyll of the leaves.

Salt gland. Source: umpedeque.com.br

Its function is the excretion of salt in plants that grow in saline environments, such as marine mangroves that absorb salts from the water. Through these glands a unidirectional flow is originated that allows to eliminate the excess of ions of potassium, salt, calcium and chlorine.

Osmophores

Osmophores are glands that eliminate or expel highly volatile oils that cause the smell of flowers. In some species, these oils are formed in the vacuoles of the cells of the epidermis and mesophyll of the petals.

Hydatodes

Hydatodes are a type of stoma that secrete aqueous solutions through a process called guttation. This process occurs when the plants favor minimal perspiration, due to the humidity conditions of the soil.

Nectaries

Nectaries are specialized glands that secrete a sugary solution or nectar, essentially made up of glucose, sucrose, fructose, maltose and melobiose. They are cells of epidermal tissue differentiated into secretory tissue or nectariferous trichomes, located in the cuticle of leaves and flowers.

Nectaries. Source: Frank Vincentz, from Wikimedia Commons

References

1. Plant Excretions (2013) Natural Sciences. Recovered at: webnode.es
2. Epidermis (2013) Morphology of Vascular Plants. Recovered at: biologia.edu.ar
3. García Bello Francisco J. (2015) Secretion Tissues. Recovered at: euita.upv.es
4. Excretion in Plants (2018) Aragonese E-ducative Platform. Recovered at: e-ducativa.catedu.es
5. Noguera Hernández A., & Salinas Sánchez M. (1991). Metabolism of the Individual. Biology II, Colegio de Bachilleres.