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Ground tissue
Ground tissue
Ground tissue
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Flax stem cross-section:

The ground tissue of plants includes all tissues that are neither dermal nor vascular. It can be divided into three types based on the nature of the cell walls. This tissue system is present between the dermal tissue and forms the main bulk of the plant body.

  1. Parenchyma cells have thin primary walls and usually remain alive after they become mature. Parenchyma forms the "filler" tissue in the soft parts of plants, and is usually present in cortex, pericycle, pith, and medullary rays in primary stem and root.
  2. Collenchyma cells have thin primary walls with some areas of secondary thickening. Collenchyma provides extra mechanical and structural support, particularly in regions of new growth.
  3. Sclerenchyma cells have thick lignified secondary walls and often die when mature. Sclerenchyma provides the main structural support to the plant.[1]
  4. Aerenchyma cells are found in aquatic plants. They are also known to be parenchyma cells with large air cavities surrounded by irregular cells which form columns called trabeculae.
Cross section of a leaf showing various ground tissue types
Cross section of a leaf showing various ground tissue types

Parenchyma

[edit]

Parenchyma is a versatile ground tissue that generally constitutes the "filler" tissue in soft parts of plants. It forms, among other things, the cortex (outer region) and pith (central region) of stems, the cortex of roots, the mesophyll of leaves, the pulp of fruits, and the endosperm of seeds. Parenchyma cells are often living cells and may remain meristematic, meaning that they are capable of cell division if stimulated. They have thin and flexible cellulose cell walls and are generally polyhedral when close-packed, but can be roughly spherical when isolated from their neighbors. Parenchyma cells are generally large. They have large central vacuoles, which allow the cells to store and regulate ions, waste products, and water. Tissue specialised for food storage is commonly formed of parenchyma cells.

Parenchyma cells have a variety of functions:

  • In leaves, they form two layers of mesophyll cells immediately beneath the epidermis of the leaf, that are responsible for photosynthesis and the exchange of gases.[2] These layers are called the palisade parenchyma and spongy mesophyll. Palisade parenchyma cells can be either cuboidal or elongated. Parenchyma cells in the mesophyll of leaves are specialised parenchyma cells called chlorenchyma cells (parenchyma cells with chloroplasts). Parenchyma cells are also found in other parts of the plant.
  • Storage of starch, protein, fats, oils and water in roots, tubers (e.g. potatoes), seed endosperm (e.g. cereals) and cotyledons (e.g. pulses and peanuts)
  • Secretion (e.g. the parenchyma cells lining the inside of resin ducts)
  • Wound repair [citation needed] and the potential for renewed meristematic activity
  • Other specialised functions such as aeration (aerenchyma) provides buoyancy and helps aquatic plants float.
  • Chlorenchyma cells carry out photosynthesis and manufacture food.

The shape of parenchyma cells varies with their function. In the spongy mesophyll of a leaf, parenchyma cells range from near-spherical and loosely arranged with large intercellular spaces,[2] to branched or stellate, mutually interconnected with their neighbours at the ends of their arms to form a three-dimensional network, like in the red kidney bean Phaseolus vulgaris and other mesophytes.[3] These cells, along with the epidermal guard cells of the stoma, form a system of air spaces and chambers that regulate the exchange of gases. In some works, the cells of the leaf epidermis are regarded as specialised parenchymal cells,[4] but the modern preference has long been to classify the epidermis as plant dermal tissue, and parenchyma as ground tissue.[5]

Shapes of parenchyma:

  • Polyhedral (found in pallisade tissue of the leaf)
  • Spherical
  • Stellate (found in stem of plants and have well-developed air spaces between them)
  • Elongated (also found in pallisade tissue of leaf)
  • Lobed (found in spongy and pallisade mesophyll tissue of some plants)

Collenchyma

[edit]
Cross section of collenchyma cells

Collenchyma tissue is composed of elongated cells with irregularly thickened walls. They provide structural support, particularly in growing shoots and leaves (as seen, for example, the resilient strands in stalks of celery). Collenchyma cells are usually living, and have only a thick primary cell wall[6] made up of cellulose and pectin. Cell wall thickness is strongly affected by mechanical stress upon the plant. The walls of collenchyma in shaken plants (to mimic the effects of wind etc.), may be 40–100% thicker than those not shaken.

There are four main types of collenchyma:

  • Angular collenchyma (thickened at intercellular contact points)
  • Tangential collenchyma (cells arranged into ordered rows and thickened at the tangential face of the cell wall)
  • Annular collenchyma (uniformly thickened cell walls)
  • Lacunar collenchyma (collenchyma with intercellular spaces)

Collenchyma cells are most often found adjacent to outer growing tissues such as the vascular cambium and are known for increasing structural support and integrity.

The first use of "collenchyma" (/kəˈlɛŋkɪmə, kɒ-/[7][8]) was by Link (1837) who used it to describe the sticky substance on Bletia (Orchidaceae) pollen. Complaining about Link's excessive nomenclature, Schleiden (1839) stated mockingly that the term "collenchyma" could have more easily been used to describe elongated sub-epidermal cells with unevenly thickened cell walls.[9]

Sclerenchyma

[edit]

Sclerenchyma is the tissue which makes the plant hard and stiff. Sclerenchyma is the supporting tissue in plants. Two types of sclerenchyma cells exist: cellular fibers and sclereids. Their cell walls consist of cellulose, hemicellulose, and lignin. Sclerenchyma cells are the principal supporting cells in plant tissues that have ceased elongation. Sclerenchyma fibers are of great economic importance, since they constitute the source material for many fabrics (e.g. flax, hemp, jute, and ramie).

Unlike the collenchyma, mature sclerenchyma is composed of dead cells with extremely thick cell walls (secondary walls) that make up to 90% of the whole cell volume. The term sclerenchyma is derived from the Greek σκληρός (sklērós), meaning "hard." It is the hard, thick walls that make sclerenchyma cells important strengthening and supporting elements in plant parts that have ceased elongation. The difference between sclereids is not always clear: transitions do exist, sometimes even within the same plant.

Fibers

[edit]
Cross section of sclerenchyma fibers

Fibers or bast are generally long, slender, so-called prosenchymatous cells, usually occurring in strands or bundles. Such bundles or the totality of a stem's bundles are colloquially called fibers. Their high load-bearing capacity and the ease with which they can be processed has since antiquity made them the source material for a number of things, like ropes, fabrics and mattresses. The fibers of flax (Linum usitatissimum) have been known in Europe and Egypt for more than 3,000 years, those of hemp (Cannabis sativa) in China for just as long. These fibers, and those of jute (Corchorus capsularis) and ramie (Boehmeria nivea, a nettle), are extremely soft and elastic and are especially well suited for the processing to textiles. Their principal cell wall material is cellulose.

Contrasting are hard fibers that are mostly found in monocots. Typical examples are the fiber of many grasses, Agave sisalana (sisal), Yucca or Phormium tenax, Musa textilis and others. Their cell walls contain, besides cellulose, a high proportion of lignin. The load-bearing capacity of Phormium tenax is as high as 20–25 kg/mm², the same as that of good steel wire (25 kg/ mm²), but the fibre tears as soon as too great a strain is placed upon it, while the wire distorts and does not tear before a strain of 80 kg/mm². The thickening of a cell wall has been studied in Linum.[citation needed] Starting at the centre of the fiber, the thickening layers of the secondary wall are deposited one after the other. Growth at both tips of the cell leads to simultaneous elongation. During development the layers of secondary material seem like tubes, of which the outer one is always longer and older than the next. After completion of growth, the missing parts are supplemented, so that the wall is evenly thickened up to the tips of the fibers.

Fibers usually originate from meristematic tissues. Cambium and procambium are their main centers of production. They are usually associated with the xylem and phloem of the vascular bundles. The fibers of the xylem are always lignified, while those of the phloem are cellulosic. Reliable evidence for the fibre cells' evolutionary origin from tracheids exists.[10] During evolution the strength of the tracheid cell walls was enhanced, the ability to conduct water was lost and the size of the pits was reduced. Fibers that do not belong to the xylem are bast (outside the ring of cambium) and such fibers that are arranged in characteristic patterns at different sites of the shoot. The term "sclerenchyma" (originally Sclerenchyma) was introduced by Mettenius in 1865.[11]

Sclereids

[edit]
Main article: Sclereid
Fresh mount of a sclereid
Long, tapered sclereids supporting a leaf edge in Dionysia kossinskyi

Sclereids are the reduced form of sclerenchyma cells with highly thickened, lignified walls.

They are small bundles of sclerenchyma tissue in plants that form durable layers, such as the cores of apples and the gritty texture of pears (Pyrus communis). Sclereids are variable in shape. The cells can be isodiametric, prosenchymatic, forked or elaborately branched. They can be grouped into bundles, can form complete tubes located at the periphery or can occur as single cells or small groups of cells within parenchyma tissues. But compared with most fibres, sclereids are relatively short. Characteristic examples are brachysclereids or the stone cells (called stone cells because of their hardness) of pears and quinces (Cydonia oblonga) and those of the shoot of the wax plant (Hoya carnosa). The cell walls fill nearly all the cell's volume. A layering of the walls and the existence of branched pits is clearly visible. Branched pits such as these are called ramiform pits. The shell of many seeds like those of nuts as well as the stones of drupes like cherries and plums are made up from sclereids.

These structures are used to protect other cells.

References

[edit]

Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Ground tissue

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Ground tissue is one of the three primary tissue systems in vascular , alongside dermal and vascular tissues, and constitutes the majority of the body in , stems, and leaves by filling the spaces between these other systems. It originates from the ground meristem during primary growth and is characterized by its diverse roles in and structure, making it essential for survival and adaptation. The ground tissue system is composed of three main cell types: , collenchyma, and sclerenchyma, each with specialized features that enable distinct functions. cells, the most abundant and versatile, feature thin primary cell walls and remain alive at maturity; they conduct in green tissues like the mesophyll of leaves, store carbohydrates and water in roots and stems, and even form protective layers such as the in roots. Collenchyma cells provide flexible mechanical support in elongating regions, such as young stems and petioles, through their unevenly thickened primary walls rich in and , while staying alive to allow for growth accommodation. In contrast, sclerenchyma cells offer rigid support via thick, lignified secondary walls, but they typically die at maturity, leaving behind durable structures like fibers for tensile strength or sclereids for hardness in tissues such as nut shells or fruit stones. Overall, ground tissue's adaptability allows it to perform a broad array of functions tailored to the 's needs, from metabolic activities in soft, herbaceous organs to reinforcement in woody structures, underscoring its foundational role in and .

Introduction

Definition

Ground tissue is one of the three primary tissue systems in vascular , alongside the dermal tissue system and the system; it forms the bulk of the non-specialized interior of the body. This system occupies the space between the protective dermal layer and the conductive vascular bundles, providing structural filler and supporting fundamental cellular processes. The ground tissue system arises from the ground meristem, a layer of undifferentiated cells in the apical meristems of roots and shoots that differentiates into simple, unspecialized parenchyma-like cells. These cells are primarily thin-walled at maturity, though sclerenchyma components develop thickened, lignified walls for added rigidity. The concept of ground tissue as a distinct system was formalized in 19th-century plant anatomy by Joseph Hanstein through his histogen theory, which proposed three initial cell layers (histogens) in meristems that give rise to the dermal, ground, and vascular systems, with the ground histogen (periblem) filling the central body mass. This framework highlighted the ground tissue's role in comprising the plant's internal volume beyond specialized coverings and conduits.

Origin and Development

The in originates embryologically from the ground meristem, one of the three primary meristems that emerge during early embryo development in the apical regions of shoots and roots. In the embryo, for instance, ground tissue identity is first specified from totipotent precursor cells around the 16-cell stage, progressing through the globular stage where auxin-mediated signaling, particularly via the ARF5/ , cell-autonomously initiates the ground tissue lineage through asymmetric cell divisions. These divisions establish the foundational layers of ground tissue, distinguishing it from the protoderm (which forms the dermal system) and procambium (which develops into vascular tissues). During primary growth, the ground meristem, located between the protoderm and procambium in developing tissues, undergoes rapid mitotic cell divisions at the shoot apical meristem (SAM) and root apical meristem (RAM). These divisions produce a cohort of undifferentiated cells that subsequently differentiate into ground tissue through a series of histological changes, including isotropic expansion where cells grow uniformly in all directions to fill space, followed by anisotropic elongation that contributes to organ lengthening. As differentiation advances, cells deposit additional cell wall materials, leading to thickening that provides structural integrity while maintaining the tissue's primary functions. This process transitions meristematic cells from a totipotent state—capable of dividing in multiple planes—to mature, specialized ground tissue cells integrated into the body. While ground tissue primarily arises from primary meristems during apical growth, it can also form secondarily from lateral meristems such as the and in woody plants, though this contributes less extensively to ground tissue volume compared to primary development. In , cells derived from these cambial layers undergo similar division and differentiation patterns, incorporating isotropic expansion and wall modifications to support radial thickening of stems and . Microscopically, these stages are observable as clusters of small, densely cytoplasmic meristematic cells evolving into larger, vacuolated cells with reinforced walls, ensuring the continuity of the ground tissue system throughout the plant's lifecycle.

Locations and Distribution

In Stems

In herbaceous stems, ground tissue primarily consists of cells that form the central and the peripheral cortex, providing structural support, storage, and metabolic functions while surrounding the vascular bundles. The occupies the interior region of the stem, filling the space inside the vascular tissue, whereas the cortex lies between the vascular bundles and the , aiding in nutrient storage and in some species. In dicotyledonous herbaceous stems, the vascular bundles are organized in a ring-like arrangement around the periphery of the , with ground tissue comprising the central and the outer cortex. By contrast, in monocotyledonous herbaceous stems, vascular bundles are scattered randomly throughout the ground tissue, lacking a distinct separation between and cortex, which allows for flexible distribution of support and transport elements. In woody stems, secondary growth driven by the expands the stem's girth, where ground tissue persists as the central and contributes to ray , which forms radial sheets of cells extending from the through the secondary . During this process, the expansion of secondary and compresses the primary ground tissue of the cortex outward toward the bark, often reducing its thickness and functionality over time, while the remains relatively intact in the stem center. Ray parenchyma, as a component of ground tissue, facilitates lateral of water, nutrients, and photosynthates across the wood. A notable example occurs in (Quercus spp.) stems, where medullary rays—broad extensions of ground tissue —form multiseriate and compound structures up to 2-6 cells wide, primarily for storing assimilates from leaves and influencing local growth patterns.

In Roots

In primary roots, the ground tissue primarily occupies the cortex, which lies between the epidermis and the endodermis, and in some cases, a small central pith within the stele excluding the vascular tissues. The cortex consists mainly of parenchyma cells that facilitate water and nutrient absorption through radial transport pathways and serve as a site for starch storage, supporting root growth and plant metabolism. The endodermis, a specialized layer of parenchyma cells with the , serves as a selective barrier regulating apoplastic flow into the vascular stele, while the cortex parenchyma dominates the ground tissue composition in this region. In storage roots, such as those of carrots (), the ground tissue undergoes significant thickening through cell expansion and proliferation in the cortex, enabling substantial reserves like sugars and starches to support future growth or . This adaptation contrasts with typical primary roots by prioritizing storage over extensive absorption, with the enlarged cortex forming a large proportion of the root's volume in such modified structures. In adventitious roots of wetland plants, ground tissue features specialized , a form of with extensive intercellular air spaces, which facilitates oxygen from aerial shoots to submerged in low-oxygen soils. This adaptation, often forming lysigenous or schizogenous spaces in the cortex, can occupy 30-60% of root volume and is crucial for maintaining aerobic respiration in hypoxic environments, as seen in species like those in the family.

In Leaves

In leaves, the primary ground tissue is the mesophyll, which is specialized for and occupies the interior space between the upper and lower . This tissue is typically divided into two distinct layers in dicotyledonous : the palisade mesophyll, consisting of elongated, columnar cells densely packed with chloroplasts located near the upper to maximize capture, and the spongy mesophyll, composed of irregularly shaped cells with numerous air spaces situated toward the lower to facilitate . The mesophyll ground tissue is arranged around the leaf's vascular bundles, or veins, filling the spaces between them and providing structural continuity while optimizing physiological functions. In particular, the air spaces within the spongy mesophyll enhance diffusion of to photosynthetic cells and release of oxygen, supporting efficient during . This arrangement integrates the ground tissue with the vascular system, ensuring and to photosynthetic sites without compromising the leaf's overall compactness. In succulent adapted to arid environments, the ground tissue in leaves is often thickened with extensive cells dedicated to , enabling survival during prolonged droughts. For example, in cacti, leaves are reduced and modified into spines for defense, shifting primary water storage to the succulent stem's ground tissue, while non-cactus succulents like those in the genus retain fleshy leaves with hydrated . This adaptation minimizes while maintaining photosynthetic capacity. A notable variation occurs in C4 plants, particularly many monocotyledons, where the ground tissue forms Kranz anatomy characterized by a wreath-like bundle sheath of larger, chloroplast-rich cells surrounding each , distinct from the surrounding mesophyll. This specialized ground tissue arrangement concentrates CO2 in the bundle sheath for the , improving in hot, dry conditions compared to the more uniform mesophyll in typical dicot leaves.

Types

Parenchyma

Parenchyma cells are the most abundant and versatile type of cells within ground tissue, characterized by their thin primary cell walls composed primarily of , , and . These cells remain alive at maturity, featuring a large central that occupies much of the cell volume and, in some cases, chloroplasts for photosynthetic activity. Their includes a prominent nucleus and , with the cells typically exhibiting an isodiametric shape, though they can vary in form depending on their location and function. Parenchyma exhibits several specialized variations adapted to specific roles in plants. Aerenchyma consists of parenchyma with extensive intercellular air spaces, providing buoyancy and facilitating gas exchange in aquatic or wetland environments, as seen in plants like water hyacinth (Eichhornia crassipes). Chlorenchyma, rich in chloroplasts, is specialized for photosynthesis and is prominent in leaf mesophyll and green stems. Storage parenchyma contains amyloplasts for starch accumulation, serving as a nutrient reserve in structures such as roots of Ranunculus. Idioblasts are isolated parenchyma cells differentiated for unique contents, such as calcium oxalate crystals (raphides) that deter herbivores through mechanical irritation and enzymatic action, as in Dieffenbachia. The primary functions of parenchyma center on metabolic activities, including photosynthesis in chlorenchyma where chloroplasts convert light energy into . These cells also enable storage of water, nutrients, and carbohydrates, supporting plant growth and survival during stress. Parenchyma contributes to through its totipotent nature, allowing to regenerate tissues in response to injury, as demonstrated in vegetative propagation from stem cuttings. Additionally, certain parenchyma forms part of glandular tissues for of oils, resins, or other compounds involved in defense and communication. Parenchyma comprises the bulk of ground tissue volume in most , forming the foundational matrix in various organs.

Collenchyma

Collenchyma is composed of living, elongated cells characterized by unevenly thickened primary cell walls, with extra deposits of and concentrated at the cell corners, and lacking secondary walls or lignification. These thickenings occur in alternating layers that enhance flexibility while providing strength, distinguishing collenchyma from more rigid tissues. The cells retain their at maturity, allowing for continued metabolic activity and adaptability during growth. In young, elongating parts, collenchyma typically forms a continuous layer in the hypodermis just beneath the of stems and petioles, or as strands along the veins of leaves, where it supports tensile forces without impeding expansion. This positioning near the periphery maximizes its role in reinforcing areas prone to mechanical stress during early development. Collenchyma is generally absent from roots, except in some aerial varieties, and diminishes in mature woody tissues as replaces it with more permanent support structures. The primary function of collenchyma is to offer flexible mechanical support to growing organs, enabling resistance to bending and compressive forces while permitting elongation and preventing tissue rupture. This is achieved through the dynamic nature of its pectin-rich walls, which can thicken in response to environmental stimuli like wind, and the cells' ability to stretch without losing viability. Unlike adjacent cells focused on metabolic roles, collenchyma's specialized wall composition prioritizes tensile strength in transitional growth phases. A prominent example of collenchyma is the fibrous, stringy bundles in stalks ( graveolens petioles), which provide the characteristic crunch and flexibility under stress. These strands illustrate collenchyma's role in herbaceous , where it supports non-woody structures during active growth.

Sclerenchyma

Sclerenchyma consists of elongated or variably shaped cells that provide rigid mechanical support to plant organs due to their heavily lignified secondary cell walls. These cells are typically dead at maturity, lacking protoplasm and intercellular spaces, which allows them to withstand tensile and compressive forces effectively. The deposition of lignin in the secondary walls imparts a hardened, impermeable quality, resulting in narrow lumens that further enhance structural integrity. Sclerenchyma is divided into two main subtypes: fibers and sclereids, each adapted for specific supportive roles within ground tissue. Fibers are long, slender, and often parallel-arranged cells with tapered ends, optimized for tensile strength; they commonly form bundles in the cortex or around vascular tissues, as seen in the bark of elderberry (). In contrast, sclereids are shorter, variably shaped cells—ranging from branched, fiber-like forms to compact, stone-like structures—that provide localized hardness; examples include the gritty stone cells in pear fruit (Pyrus spp.) and the tough sclereids in coconut shells (Cocos nucifera). The primary function of sclerenchyma is to offer permanent mechanical support to mature parts, preventing deformation under mechanical stress or by maintaining structural rigidity. This hardness also serves protective roles, deterring herbivores and pathogens through tough barriers, as in seed coats or nutshells. Additionally, certain types contribute to water conduction by reinforcing vascular elements, though their main role remains supportive. Sclerenchyma cells are distributed throughout ground tissue in mature stems, , leaves, and fruits, often increasing in abundance with age as tissues lignify for long-term stability. Fibers predominate in elongating regions like stem cortex or veins, while sclereids are scattered or clustered in protective layers, such as hulls of pods or water lily leaves.

References

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