Parenchyma Cells – Biological Significance and Functions
The word parenchyma means literally “poured in beside” and suggests the gap-filling capacity visualized by earlier histologists, who thought that the cells occupying the spaces between the more solid components of plant parts had been formed merely to fill them. This is far from the truth, since mature parenchyma is as much a functional tissue as any other. It can also be regarded as the foundation of the plant body, since all cells produced from the meristems and regions of regenerations go through stages of development when they possess characters similar to parenchyma cells. In the evolutionary series, it is certain that this cell form has come first; primitive plant structure bears testimony to this. Functionally, the various forms of parenchyma are capable of a wide range of purposes such as photosynthesis, food storage and secretion; all metabolic activity occurs in their protoplasts.
It is now usual to regard any mature but relatively undifferentiated or unspecialized cell as parenchyma, in contrast to the much more highly specialized epidermal, mechanical or conducting cells. Nevertheless, parenchyma may show specialization according to its position in the plant. For example, inner leaf tissues are highly specialized for photosynthesis and root cortex for food storage. At the same time, parenchyma cells retain their ability to change their nature according to the needs of the plant, and this they frequently do.
Tissue Formation by Parenchyma Cells
In the higher plants, parenchyma cells usually form fairly clearly-defined tissues, although they may be associated with groups of more specialized cells to form mixed tissues. This condition occurs in the vascular regions, where parenchyma cells form vertical and horizontal strands among the conducting elements. Pith and cortex of stems and roots, mesophyll of leaves, succulent or fleshy storage tissues of any sort are all examples of regions chiefly parenchymatous. However, where there is no clear distinction between parenchyma and other kinds of cell, since variation in shape, size and wall structure make it difficult make it to draw a sharp dividing line.
Parenchyma cells are extremely variable in shape. We may see them roughly isodiametric with many flattened or rounded faces. Alternatively, they may be considerably elongated along one axis to conform to the pattern of the so-called prosenchyma, a term applied to any elongated cells with tapering ends. The walls may be variously lobed or in fact, so great is the variation that it is nearly impossible to generalize. However, assuming that the tissues is derived from cubical, box –like initial cells, all equal in size and all differentiating such cell in the ideal case would possess fourteen faces, eight hexagonal in outline and six oblong. The condition under which adjacent cells differentiate rarely remain constant. The original cells are most often of unequal size; they are irregularly spaced, and they differentiate at different rates. Furthermore, intercellular space are of frequent occurrence in parenchyma, and they tend to cut down the number of cell contacts so that less than fourteen faces occur. Thus rarely, if ever, is the ideal shape achieved. However, in homogeneous parenchyma, the than the twelve which would occur if the initial cells were spherical. All parenchyma cells have a common characteristic. They all possess living protoplasts which have special feature related to the main function of the cell. For example, the actively photosynthesizing leaf mesophyll contain abundant chloroplasts. This feature is so characteristic that the tissues is frequently termed chlorenchyma. Storage cell contain various products of metabolism. Protoplasts of parenchyma cells are commonly interconnected from cell to cell by plasmodesmata through simple pits in the walls.
Parenchyma cell walls are most commonly thin, primary, cellulose walls with frequently pit markings. There can be considerable thickening of these primary walls however, particular in some storage organs, where hemicelluloses may be deposited in the cell walls as, e.g. the endosperm of the date palm. In parenchyma cells associated with vascular tissues, it is not unusual to find the walls with a secondary layer of lignified material forming sclerotic parenchyma, but the living contents distinguish them from the very similar sclerenchyma cells.
Parenchyma cells worthy of special mention are those which occur as the innermost layer of stem and root cortex in pterdophytes and spermatophytes, forming the endodermis. Endodermal cells are some-what elongated in the longitudinal axis. In the young endodermis of pteridophytes and angiosperms, only the radial walls are thickened by a narrow band of material composed of lignin or suberin or some related fatty substance. This band, the casparian band, completely encircles the cell in the tangential plane. A few cells remain unthickened and are called passage cells. In the transverse section, the bands appear as thickened zones on the radial walls. In older regions of an endodermis, the thickening may extend to other parts of the cells. In monocotyledonous angiosperm roots the endodermis is characterized by having thick cutinized and sometimes partly lignified walls. This thickening may be all over the cell or restricted to the radial and inner tangential walls only. Endodermal cell walls are considered to be impervious to water in the radial direction, but the occasional unthickened passage cells break the continuity. In some stems, the more characteristics endodermis may be replaced by a layer of cells containing prominent starch grains forming the starch sheath.
Another parenchyma cell layer in some stems and roots is the pericycle, a layer of cells inner to the endodermis, and clearly distinguishable from it by possessing no thickening of the walls. The pericycle may be regarded as the most external of the centrally-placed vascular tissues. Its special characteristic is that the cells retain their potentiality for further division after maturity, and certain outgrowth of roots and shoots have their origin in the in the pericycle. In appearance the cells are not much different from those of cortex or pith.
Other specialized parenchyma cells forms the companion cells in angiosperm phloem tissue.
Parenchyma cells make up the major portion of fundamental or ground tissues. These are living, thin-walled and simple in structure. They perform several vital functions.
They can be rounded, oval or elongated. They have small intercellular spaces and are tightly packed. Their cell walls are thin and a thick fluid fills their cytoplasm. They also contain a nucleus. They can be found in all parts of plants.
Circular Parenchyma Cells
Parenchyma cells make up ground tissue that separates vascular and dermal tissues of a plant. It stores food materials such as starch, hormones, fat and nutrients. It also helps in wound healing and storing waste material such as ergastic substances. In addition, it stores a lot of water in its large central vacuole. This helps the plant in growth and development.
Parenchymal cells can be either structural or vascular. Structural parenchyma cells are usually polygonal in shape and tightly packed without any intracellular space. They are found in the pith and cortex of stems and roots, flesh of succulent fruits, mesophyll of leaves and seed endosperm. Moreover, it can also be present as a component of xylem and phloem.
On the other hand, vascular parenchyma is made of loosely packed cells with more intercellular spaces. It is found in the perivascular and medullary zones of primary vascular tissues in monocots, as well as in the cortical meristem of dicots. It is also seen in the sclerenchyma cells of secondary vascular tissues.
It is also important for nutrient transport, which takes place through a series of thin-walled tubes called phloem vessels. These are surrounded by the perivascular parenchyma, which is composed of tapering, elongated and cylindrical cells. The elongated cells carry dense cytoplasm and can form plasmodesmata associations with adjacent cells via pits in their walls. This enables the long-distance transmission of solutes such as sucrose.
In addition to storing food, vascular parenchyma also carries water and minerals from one part of the plant to another. This is done through a process known as radial conduction, which is carried by the phloem parenchyma. Moreover, it can be accompanied by xylem parenchyma to increase the rate of nutrient transport.
It is also important for growth of the plant, which happens through a series of cell divisions. This process is controlled by the meristem, which is a type of parenchyma cell. The meristem is located in the mesophyll of the plant and contains chloroplasts, which perform photosynthesis. It is responsible for the green color of a plant and gives it a distinctive appearance.
Aerenchyma
The aerenchyma is one of the major components of ground tissue in plants. It consists of simple, living and undifferentiated cells. It performs various functions. These include transfer of solutes over short distances, storage, and water and food reservoirs. It also helps in the repair of wounds. In the case of a wound, parenchyma cells become stimulated to divide and differentiate into meristematic cells. This is what causes the healing of plants from a giant tree to a blade of grass.
Aerenchyma is spongy tissue with large intercellular spaces adapted for internal circulation of gases. It is found in the roots and stems of aquatic plants. It is essential for the growth of these plants. It allows oxygen to be conducted from the soil to the plant, and removes gases such as carbon dioxide and ethylene. It is also important for the plant’s buoyancy.
In some wetland plants, aerenchyma is known as “schizogenous” because it forms without cell death. This occurs because the cells enlarge differentially and create ordered gas spaces. It is not the same as sclerenchyma, which is a more dense and intractable tissue.
It is believed that the aerenchyma provides the vascular tissues with the water and minerals they need for growth. It is also thought to help them withstand water-stress conditions. It also serves as a water and nutrient reserve for the plant. In addition, it can store starch grains.
Aerenchyma is also important in the growth of sugarcane and corn. It enables the plants to survive in higher water tables and slows the degradation of muck organic soils. Agricultural scientist W. Doral Kemper, a retired ARS national program leader for soil management, has participated in digging expeditions in which his colleagues found that aerenchyma enables root tips to snorkel air from above-water parts of the plant. The dark openings in this cross section of an eastern gamagrass root are aerenchyma. Kemper’s lab is currently studying the aerenchyma of sugarcane and corn to learn more about its role in the roots. He hopes to breed crops with more aerenchyma, so that they can grow in wetter conditions.
Chlorenchyma
The chlorenchyma is a type of parenchyma that contains chloroplasts and helps plants perform their photosynthetic function. It is mainly found in the mesophyll area of leaves and some green stems. Chlorenchyma cells are thin-walled and are typically arranged in rows. They are the principal photosynthesizing cells in most plants. Beneath the chlorenchyma is a layer of spongy mesophyll tissue, which consists of loosely packed, thin-walled cells that are separated by lacunae (intercellular spaces). The cells in spongy mesophyll are able to circulate carbon dioxide, which helps make photosynthesis possible.
The thallose liverworts of the order Marchantiales are regarded as the closest analog to flowering plants among bryophytes, and their morphology is particularly well-suited for studying photosynthesis and water movement in vivo. They have a unique structure, including a convoluted internal layer of chlorenchyma cells, that is similar to the outer pericarp of vascular plants. Moreover, remobilization processes may feed the pericarp during the formation of the embryo sac, as evidenced by the graduated expression of the cell-wall-bound invertase HvCWINV2 at 2 days after fertilization (DAF).
To examine inter-tissue water transfer during drought in the cactus Hemiepiphyticus undatus, sections from the chlorenchyma and the water-storage parenchyma were perfused with distilled water. The average osmotic pressure of the perfused cells was measured using an electronic osmometer. The results revealed that the water-storage parenchyma had a lower average osmotic pressure than the chlorenchyma. This difference is attributed to the fact that the cell walls of the water-storage parenchyma are more flexible than those of the chlorenchyma.
The average water potential of the chlorenchyma was -0.3 MPa higher than the osmotic pressure of the sclerenchyma, indicating that they were more resistant to dry conditions. When the sclerenchyma cells were subjected to 6 weeks of water deprivation, their water potential changed by only 24 %. This result indicates that the water-storage parenchyma is able to transfer some of its water to the adjacent chlorenchyma during drought. A similar finding was observed in the sclerenchyma of Hemiepiphyticus undatus after it had been subjected to an 8-week drought.
Storage parenchyma
Parenchyma cells are used to store different substances like water, proteins and starches. It also stores waste products and ergastic substances. These cells form a mass into which more specialized cells like vascular tissues and seeds are imbedded. These cells are soft, thin-walled and loosely packed.
Mesophyll parenchyma is found in all green parts of the plant and contains chloroplasts, which performs photosynthesis. This type of parenchyma is present in leaves, stems, roots and flowers. This kind of parenchyma cells are also responsible for the transport of food to the cells in a plant.
Other kinds of parenchyma include xylem parenchyma and ray parenchyma. Xylem parenchyma consists of thin-walled, cylindrical and tapering cells. It stores food products like fats, starches and some other materials such as resins, mucilage and latex. It also helps in radial conduction of water. Its cell wall is made up of cellulose and is a good storage material in water stress conditions.
A major function of ray parenchyma is to store starch grains in the medullary ray of the primary vascular tissue of the stem. These cells can be found in seed plants and some woody perennials. They also serve as a reservoir for minerals and water. They are able to support new shoot growth and root development during winter in stems of some plants. They can also initiate regrowth of new shoots in spring in monocotyledons.
Parenchyma cells that are located adjacent to a vessel in the vascular tissue may ‘grow into’ the vessel. This is called ingrowth or encroaching. It can occur during normal heartwood formation or due to injury. This process alters the permeability of the wood and lowers its water uptake. It also slows drying and makes it difficult to treat with preservatives. In woody species, these ingrowing parenchyma cells may also manufacture extractives which color the wood, coat cell walls and passages between cells, and reduce permeability. These substances are referred to as tyloses and they contribute to a slow vapor flow, which in turn causes slower drying of the wood and a reduced ability to absorb preservatives.
