Why are parenchyma simple cells

In addition, J. v. SACHS:

"The totality of all tissue masses, which are enclosed by the skin tissue (note: the epidermis) and traversed by the vascular bundles, I summarize under the term basic tissue, as I first characterized it in the 1st edition of my textbook 1868. With younger ones Organs, still juicy, only covered by the epidermis, whose vascular bundles have not yet been deformed by subsequent growth in thickness, in organs in general, in which the formation of real wood and secondary bark has not yet occurred, the main mass consists of basic tissue the same perhaps when looking at an apple whose entire edible substance consists of it ... ... The most common and, as one may well assume, the most original form of the ground tissue is the ordinary, thin-walled parenchyma. "

Parenchymatous cells are always alive, mostly isodiametric, less often elongated. The pulp of the shoots, the storage tissue of the fruits, seeds, the roots and other subterranean organs are to be regarded as parenchyma as well as the mesophyll (assimilation tissue of the leaves). Because of its fundamental importance for assimilation and thus for the entire nutrition of the plant, it is advisable to deal with the mesophyll in a separate section (following the discussion of the less specialized parenchyma).

Parenchymatous cells and relatively large intercellular cells in a stem cross-section of Geum urbanum . Fluorochroming with coriphosphine. The central lamella appears here as a red fluorescence.

Often the parenchyma apparently only has a simple filling function. As the name basic tissue already suggests, it represents the mass of cells in which specialized tissues such as conductive tissue and often also the seeds are embedded.

Parenchymal cells are neither morphologically nor physiologically specialized. They often contain chloroplasts, sometimes, especially where they have developed in the absence of light, leucoplasts or incompletely structured chloroplasts. Although no or only sporadic cell divisions take place in differentiated parenchyma, the cells retain their readiness to divide, which is expressed in the fact that they make up the majority of the cells involved in wound healing and the regeneration of plant parts. So you could say that the cells are on hold. They form a cell pool (a cell store) that can be activated when required, i.e. during normal ontogenesis and also on exceptional external occasions.

We have characterized the meristem as the dividing tissue of the plants. The parenchyma could accordingly be described as a starting tissue from which the most diverse cell types are derived during ontogenesis. The importance of the position of a cell in relation to the rest is pointed out in the topic of cells and tissues; it depends on whether a cell divides or not (position information). In parenchymatic cells, too, the position of a cell determines its future fate and the direction in which the cell (or a group of neighboring cells) will develop.

Parenchymatic tissues (e.g. from the pulp of the shoot) can be cultivated on suitable synthetic nutrient media (nutrient media), thereby stimulating the cells to divide. The division capacity is maintained for decades if parts of the cultures are transferred to a fresh medium at regular intervals. The cells remain in a largely unspecialized state. They form aggregates that are referred to as calluses (sing. Callus); the cultures themselves are called tissue or callus cultures. If you keep them in the dark, the calli remain colorless; they usually turn green when exposed to light. By adding suitable growth substances (phytohormones), shoot and / or root formation can be induced.

A large number of specific substances are produced in the metabolism of plants, some of which are stored and used at a later point in time if necessary. Again, it is predominantly parenchymatic cells in various aboveground and subterranean organs that serve as a depot. We will deal extensively with the chemistry of the stored substances later. It should only be noted here that a basic distinction must be made between small and large molecules (macromolecules). In addition to the inorganic ions, organic acids (or their salts), sugars, a number of nitrogenous compounds (e.g. amino acids, alkaloids) and others are counted among the small ones, and the macromolecules include starch and proteins. Small molecules are usually stored in the vacuoles in dissolved form. Occasionally they crystallize out and the crystals are deposited either in the vacuole or in the plasma, depending on the type. Macromolecules rarely reach the central vacuole. They are deposited in the plasma as microscopically visible aggregates (protein bodies, starch granules).

Sugar and starch are among the most important primary products of photosynthesis. It is well known that the parenchyma of fruits (the pulp) contains a wide variety of sugars, sometimes in considerable amounts. The simultaneous storage of organic acids (in the same cells) gives them a species-specific, often sweet and sour taste. Starch deposits are found in seeds (in the endosperm, also a parenchymal tissue), in roots, tubers (dahlias, potatoes), sprouts (potatoes), etc.

Proteins are also often stored in seeds, mostly in specialized cells. In a wheat grain, for example, starch is found in the centrally located endosperm cells and in protein further out in the form of so-called aleurone grains. The proteins are storage proteins that belong to a certain protein class. Their chemical composition and structure is species-specific.

When the seeds germinate, the macromolecules (reserve substances) stored in the endosperm are partially degraded and serve to build up the seedling. Once its ingredients have been mobilized, this tissue usually dies.

In addition to the compounds just mentioned, parenchymal cells store large amounts of water. Particularly evident in the tissues of succulent (thick-fleshed) plants: cacti, Aloe, agave, Aizoaceae (= midday flowers). A direct (linear) correlation between the percentage of parenchymal tissue and water content was determined in the individual sections of a bamboo stalk.

However, certain parts of the plants in our latitudes also contain more water than the others: these include, for example, the pulp, the onions, the buds, all "fleshy" thickenings of above-ground parts, etc.

The capacity to store water in the vacuoles depends on the molarity of the substances dissolved in them. One speaks of the osmotic value (osmolarity).

In the previous section we got to know the vacuoles of parenchymal cells as depots for a number of small molecules. It follows from the physicochemical condition just postulated that precisely such cells are predestined for a high rate of water absorption. You can tell that just by the fact that ripe fruits are always plump; in overripe the high osmotic pressure can even lead to their bursting.

In most parenchymal tissues, especially those with a storage function, there are only small, sometimes even no intercellular tissues. This can be explained on the one hand by the high osmotic pressure (= turgor) of the individual cells, on the other hand by their thin cell walls, which offer little resistance to the expansion of the cell content and thus prevent the formation of intercellular cells in the first place.

The situation is different from what has just been described in the case of a special type of parenchymal cells, which is characterized by thicker walls. When such cells diverge or the tissues tear, air-filled cavities are created. The cavities created by tearing are usually tangential or radial in the shoot and in the roots. The cause of the tearing is to be found in the differences in tension between neighboring tissues that are expanding differently. When they diverge, based on asymmetrical elongation, irregularly shaped cells develop, between which extensive intercellular systems develop. The sponge parenchyma of the leaves is an example of this. Extensive ventilation tissue can also be found in the marrow of many monocotyledons in damp locations, e.g. in the rushes. Because of the specific cell shape that occurs there, the tissue type is referred to as star parenchyma.

Star parenchyma from the pith of the stem of rushes (Juncus). Extremely large intercellulars. The parenchyma serves here as a ventilation tissue
(Photo: W. KASPRIK).

In submerged (submerged) sprouts of many angiosperms, such as the white and yellow pond rose, there are large air ducts in the parenchyma, which can be seen with the naked eye and which are separated from one another by simple cell layers. This honeycomb-like construction requires two things: Firstly, high strength with only a small amount of material. Incidentally, this principle is also used successfully in the technological field. On the other hand, the low specific weight generates buoyancy, which benefits the buoyancy of the rung.

Aerenchyme in stem cross-sections of
Hippuris vulgaris