Meristems (Meristematic Tissues)

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Meristems (meristematic tissues) (from Greek meristos — divisible) are permanently functioning plant tissues whose cells retain the ability to divide throughout the life of the organism (Evert, 2006). It is thanks to meristems that indeterminate (open) growth occurs — the formation of new cells, tissues and organs throughout ontogeny, which fundamentally distinguishes plants from most animals.

In simple terms, while in animals all organs are laid down during embryogenesis, plants continue to build them throughout their life. Meristems act as “cell factories,” laying the foundation for future organs and providing the ability to regenerate after damage (Beck, 2010). Although meristem cells constitute a negligible fraction of the total plant volume (less than 0.1%), they determine all subsequent growth and architecture (Yakovlev et al., 2008).

Evolutionary origin.

Meristems are a key evolutionary acquisition that allowed plants to colonise land and diversify into various ecological niches. In their distant ancestors — unicellular algae — growth was indeterminate but primitive: the cell simply enlarged and then divided into two. With the transition to multicellularity and, especially, to terrestrial life, the need for specialised growth centres arose. The earliest land plants (e.g., rhyniophytes) already possessed apical meristems, although less differentiated than those of modern angiosperms (Evert, 2006).

During evolution, the organisation of meristems became more complex. From a simple apical cell (as in modern ferns and horsetails), from which the entire shoot body is derived, plants transitioned to multicellular apical meristems with clear zonation — in gymnosperms and angiosperms. In parallel, lateral meristems (cambium and phellogen) emerged, allowing plants to thicken and form massive trunks and crowns, which became a decisive factor in their competition for light (Beck, 2010).

Analogues in other organisms.

Animals have no direct analogues of meristems that provide unlimited growth of the whole organism. However, functional and partly morphological similarity can be found in stem cells of multicellular animals (Evert, 2006; Aichinger et al., 2012).

Similarities: Like meristematic cells, animal stem cells (e.g., of the epidermis, intestine, bone marrow) are capable of long-term self-renewal and give rise to various differentiated cells. They also contain a nucleus and are relatively free of specialised structures.
Principal difference: Animal stem cells are generally not totipotent (cannot give rise to a whole organism), are confined to specific tissue niches, and, most importantly, overall body growth in animals is determinate and ceases after a certain age. Plant meristems, however, function throughout life, and many of their cells retain totipotency — the ability, under certain conditions (e.g., in tissue culture), to regenerate an entire plant from a single somatic cell.

Thus, meristems are a unique evolutionary innovation of plants, providing phenotypic plasticity, long-term growth, and regenerative capacity, which has made them the dominant photosynthetic organisms on the planet.

1. Functions of Meristems

Meristems are not just “places where cells divide.” They are strategic control centres for plant growth and development. Their set of functions can be divided into two main groups: growth-related (increase in size) and morphogenetic (formation of new structures and restoration). All functions are closely interconnected and ensured by the continuous operation of various types of meristems.

1.1. Ensuring primary growth (growth in length)

This function is primarily performed by apical meristems, located at the tips of shoots and roots.

Shoot apical meristem constantly produces new cells, which then differentiate into stem tissues, leaves, and flowers. This results in the shoot elongating and its architecture being shaped. In grasses and horsetails, intercalary meristems located at the bases of internodes also contribute significantly to stem elongation. Their activity allows the stem to elongate rapidly, which is important, for example, for heading (Evert, 2006; Beck, 2010).
Root apical meristem enables root growth in length. It is protected by the root cap, whose cells are also continuously renewed through division of specialised initials. The activity of the root meristem allows the plant to penetrate new soil layers in search of water and minerals (Dolan et al., 1993).

The processes associated with the activity of apical meristems and leading to the formation of primary tissues are collectively referred to as primary growth.

1.2. Ensuring secondary growth (growth in thickness)

This is the prerogative of lateral meristems — cambium and phellogen (cork cambium).

Cambium — a single-layered cylinder of meristematic cells located between the bark and the wood in stems and roots of perennial plants. Cambial cells divide periclinally (parallel to the surface), depositing derivatives inward (toward the centre of the organ), which differentiate into secondary xylem (wood), and outward, forming secondary phloem (bast). It is precisely due to the continuous work of the cambium that tree trunks and shrubs thicken annually, growth rings form, and a powerful conducting and mechanical system develops (Fischer et al., 2019; Evert, 2006).
Phellogen (cork cambium) arises in the bark or epidermis and deposits cells outward, which become cork (phellem) — a protective tissue impermeable to water and gases — and inward, into living parenchyma cells called phelloderm. The combination of phellogen, phellem and phelloderm forms the periderm, which replaces the epidermis when stems and roots thicken (Beck, 2010).

The set of processes associated with the activity of lateral meristems is called secondary growth. Thanks to it, plants such as oaks or sequoias reach giant sizes and live for hundreds and thousands of years.

1.3. Formation of new organs

Meristems are the sole source of all plant organs: leaves, flowers, fruits, lateral shoots and roots.

Leaf and flower primordia (primordia) are initiated on the flanks of the shoot apical meristem. This is one of the earliest and most striking examples of organogenesis, determining crown architecture and the reproductive sphere (Aichinger et al., 2012).
Lateral shoots (branches) arise from axillary meristems — small groups of meristematic cells formed in the leaf axils. Sometimes they can remain dormant for a long time (dormant buds), becoming activated when the apical meristem is damaged or conditions change.
Lateral roots are initiated endogenously (inside tissues) from pericycle cells — a meristematic layer surrounding the vascular cylinder of the main root. The ability of the pericycle to form new meristems is the basis for root system branching (Dolan et al., 1993).

1.4. Regeneration and wound healing

One of the most important adaptive functions of meristems is the ability to restore lost or damaged parts. The plasticity of plant cells allows them to “remember” their meristematic past.

When the bark or wood is damaged, the cambium becomes active and forms callus outgrowths — unspecialised meristematic tissue that closes the wound. From callus cells, a new periderm can then differentiate, and sometimes wound meristems are initiated, giving rise to adventitious buds or roots (Evert, 2006).
This property is widely used in horticulture: grafting and cutting are based on the ability of the cambium of rootstock and scion to form a unified callus and restore the conducting system.
Removal of the apical meristem (e.g., by pinching) removes apical dominance, leading to the activation of dormant axillary meristems and the formation of a more branched bush (Serebryakova et al., 2006).

1.5. Callus formation

Callus is a specialised form of meristematic tissue that arises from any living, differentiated cells in response to wounding or during in vitro culture (in a test tube). Callus cells are totipotent: they can differentiate into any plant tissue and even regenerate a whole organism. This property underlies methods of clonal micropropagation, somatic hybridisation and plant genetic engineering, where callus serves as starting material for obtaining genetically identical or transgenic plants (Evert, 2006; Beck, 2010).

Thus, meristems perform not merely a growth function but an integrated role as a dynamic architectural system that builds, remodels, and repairs the entire plant body throughout its life. Understanding these functions is the foundation for practical management of growth and development in agronomy and biotechnology.

2. Cytological characteristics of meristematic cells (general structure)

Despite the diversity of meristem types (apical, lateral, intercalary), their cells exhibit remarkable similarity, reflecting their primary purpose — division and synthesis. These features are so characteristic that they allow meristematic tissue to be unambiguously identified in any section. Let us consider the main structural features of meristematic cells, typical of them during the active division stage (Evert, 2006; Beck, 2010).

2.1. Shape and size

Cells of apical meristems and their immediate derivatives have an isodiametric (approximately equal length, width and height) polyhedral shape, often with 14 faces (Serebryakova et al., 2006). However, depending on their location within the meristem and the tissue type, the shape may vary:

In the shoot apical meristem (central zone), cells are usually polyhedral, tightly packed.
In the cambium (lateral meristem), cells are prosenchymatous, i.e., highly elongated along the organ axis, spindle-shaped or tabular (Serebryakova et al., 2006; Evert, 2006).
The size of meristematic cells is generally small (from 5 to 30 μm). The small size provides a high surface-to-volume ratio, important for intensive metabolism.

2.2. Cell wall

The cell wall of meristematic cells is primary, thin (0.1–0.5 μm), non-lignified. Its basis is pectins and hemicelluloses, while the cellulose content is low (less than 30%). Such a wall is highly plastic and capable of stretching during cell growth. The absence of secondary thickenings is a crucial feature distinguishing meristems from mechanical tissues. Intercellular spaces are absent, ensuring close contact and efficient signal transmission (Serebryakova et al., 2006; Evert, 2006).

2.3. Protoplast: cytoplasm, vacuoles and organelles

Cytoplasm: Characterised by high density (dense, viscous). This is due to the huge number of ribosomes, polysomes and high synthetic activity. Under a light microscope, the cytoplasm appears homogeneous or finely granular.
Vacuoles: Numerous but very small (often invisible under light microscopy). The absence of a large central vacuole is a key difference from mature parenchyma cells. Small vacuoles do not divert resources to storage and maintain high metabolic activity.
Plastids: Present in an undifferentiated state — as proplastids. These are small (1–2 μm) colourless bodies that, depending on the cell’s function, can later turn into chloroplasts (in photosynthetic tissues), leucoplasts (in storage tissues) or chromoplasts (in coloured parts of flowers and fruits) (Serebryakova et al., 2006; Evert, 2006).
Mitochondria: Small, with simple cristae, but numerous, reflecting the high energy demand of dividing cells.
Endoplasmic reticulum (ER) and Golgi apparatus: Well developed (especially rough ER), as they ensure the synthesis and transport of proteins and polysaccharides for building new cell walls and organelles.

2.4. Nucleus and cell cycle

The nucleus in a meristematic cell is large, round, and occupies a central position. The nucleocytoplasmic ratio (ratio of nuclear volume to cell volume) is significantly higher than in differentiated cells. The nucleus contains well‑visible nucleoli, indicating intense synthesis of ribosomal RNA. Meristem cells are in active phases of the cell cycle:

A long interphase (G1, S, G2), during which DNA replication and preparation for mitosis occur.
A short mitosis (M‑phase), ensuring rapid increase in cell number.
In the central zone of the apical meristem (the quiescent centre) and in the cambium during dormancy, cells may remain arrested in the G1 phase for a long time, retaining the potential to divide when necessary (Aichinger et al., 2012; Dubrovsky & Vissenberg, 2021).

2.5. Features of different meristem types

Although the features described above are common, there are also differences:

Cells of apical meristems are the smallest, with the densest cytoplasm and small vacuoles.
Cambial cells are more vacuolated, with one or several large vacuoles, which is characteristic of lateral meristems. However, they retain the ability to divide periclinally (Beck, 2010).
Intercalary meristems of grasses are structurally similar to apical ones, but are located not at the tip, but at the bases of internodes.

Thus, the cytological portrait of a meristematic cell is that of an “eternally young”, unspecialised, highly active cell geared towards division and synthesis, which allows it to be the foundation for building the entire plant body. Understanding these cytological features is necessary for diagnosing tissue types and managing growth processes in agronomy.

3. Classification of meristems

The diversity of meristems in higher plants reflects the complexity of their organisation and the need for different types of growth. To systematise knowledge, meristems are classified according to several key criteria: by origin (primary/secondary), by position in the plant body (apical, lateral, intercalary, wound), and by activity (obligate/facultative). Such a multi‑faceted classification makes it possible to accurately describe any educational centre and understand its role in the development of a particular organ or the whole plant (Evert, 2006; Serebryakova et al., 2006).

3.1. Classification by origin

By origin, meristems are divided into primary and secondary. The key difference is from which cells they form: directly from embryonic tissues or by dedifferentiation of already mature cells.

Primary meristems

Primary meristems are established during embryogenesis and are direct descendants of the embryo cells. They retain meristematic activity throughout the plant’s life (in perennials) or for the growing season (in annuals). They include:

Apical meristems of the shoot and root (together with their derivatives — procambium, protoderm and ground meristem). The cells of these meristems are initially embryonic and never undergo full differentiation.
Intercalary meristems, although they appear later, are also primary because they arise from undifferentiated tissues left over from apical meristems at the bases of internodes (e.g., in grasses) or at leaf bases (Evert, 2006; Beck, 2010).

All tissues that form from primary meristems are called primary tissues, and the growth itself is primary growth. The primary plant body (shoot and root) consists of these tissues.

Secondary meristems

Secondary meristems (or follicular meristems) arise from differentiated, mature cells through dedifferentiation — a return to a meristematic state with loss of specialised features and resumption of division capacity. This is an evolutionarily later acquisition, characteristic primarily of seed plants (gymnosperms and angiosperms).

Classic examples of secondary meristems:

Cambium (vascular cambium). Its initial cells originate from two sources:

Fascicular cambium — a remnant of procambium between primary xylem and phloem in vascular bundles. Formally, it is a primary meristem.
Interfascicular cambium — arises by dedifferentiation of parenchyma cells of medullary rays (between bundles). This is a purely secondary meristem. Together they form a continuous cambial ring. Thus, the cambium is a tissue of mixed origin, but overall it is classified as a secondary meristem (Evert, 2006).

Phellogen (cork cambium). Arises in various layers of the bark (sometimes in the epidermis) from already differentiated collenchyma or parenchyma cells that re‑acquire the ability to divide. This is a typical secondary meristem.

Wound (traumatic) meristems. Form from living parenchyma cells around the site of injury. Their division leads to the formation of callus — a mass of unspecialised cells that then differentiates into cork or (under favourable conditions) into new organs (Evert, 2006; Serebryakova et al., 2006).

Cells of secondary meristems are generally more vacuolated and have a more elongated (prosenchymatous) shape than cells of primary apical meristems (e.g., cambial cells are highly elongated along the stem axis). However, their functional capacity to divide and produce new tissues is not inferior, and often superior, to that of primary meristems (Beck, 2010).

Understanding the origin of meristems is important for explaining the phenomenon of secondary growth (thickening), which is characteristic of trees and shrubs. It is entirely ensured by the work of secondary meristems — the cambium and phellogen. Without the ability of mature cells to return to a meristematic state, phenomena such as bark formation, wound healing and vegetative propagation (e.g., formation of adventitious roots on cuttings) would be impossible.

3.2. Classification by position in the plant

By position in the plant body, several types of meristems are distinguished: apical, lateral, intercalary, peripheral (flank) and wound (traumatic). This classification reflects the spatial organisation of growth processes and their contribution to the formation of plant architecture (Evert, 2006; Beck, 2010).

Apical meristems

Apical meristems are located at the tips of all shoots and roots (main and lateral). They provide primary growth — elongation of axes and initiation of new organs (leaves, flowers, lateral roots).

Shoot apical meristem (growth cone) — a conical or dome‑shaped structure protected by older leaf primordia. In angiosperms, it is divided into tunica (one or several layers of cells dividing anticlinally) and corpus (inner mass with random cell divisions). In gymnosperms and ferns, there is often a single large apical cell (initial) that is the source of all shoot tissues (Evert, 2006; Aichinger et al., 2012).

©Benutzer:Griensteidl commons.wikimedia.org
Public Domain
Apical meristem of the root
Micrograph of a longitudinal section of an onion (Allium) root. The root cap (Root cap) protecting the apical meristem (Apical meristem) is clearly visible, as are the zones of cell division, elongation and differentiation.

Root apical meristem is located subterminally (under the root cap). It also consists of initials, the derivatives of which form all root tissues (rhizodermis, cortex, central cylinder). An important structure is the quiescent centre — a group of slowly dividing cells in the centre of the apical meristem, which serves as a reserve for meristem restoration after damage (Dolan et al., 1993; Dubrovsky & Vissenberg, 2021).

Lateral meristems

Cork cambium (phellogen) in a cross‑section of a linden stem — commons.wikimedia.org CC BY-SA 4.0
Cork cambium (phellogen)
Micrograph of a cross‑section of a linden (Tilia) stem, showing the cork cambium layer — a secondary lateral meristem that produces cork.

Lateral meristems are located parallel to the lateral surfaces of axial organs (stems and roots), forming cylindrical layers. They provide secondary growth — increase in organ thickness. These include:

Vascular cambium (cambium) — a ring of meristematic cells between secondary xylem (wood) and secondary phloem (bast). As a result of its activity, annual growth rings of wood are formed. Cambium is present in all gymnosperms and most dicotyledonous angiosperms (Fischer et al., 2019; Evert, 2006).
Cork cambium (phellogen) — arises in the bark (sometimes in the epidermis) and deposits cells outward, forming cork (phellem), and inward — phelloderm. The combination of phellogen, phellem and phelloderm is called periderm. The periderm replaces the epidermis in ageing stems and roots, performing a protective function (Beck, 2010).

In monocotyledonous plants (e.g., palms, dracaenas), typical cambium is absent, but some species have a primary thickening meristem (atypical secondary thickening), which can also be considered a special form of lateral meristem (Evert, 2006).

Intercalary meristems

Intercalary meristems are regions of actively dividing cells inserted between mature tissues. They arise as derivatives of apical meristems, but unlike them, they have no initials of their own and eventually fully differentiate. A typical example is the bases of internodes in grasses (e.g., wheat, maize) and horsetails. It is thanks to intercalary meristems that grass stems elongate rapidly (e.g., during heading). Such meristems may remain active for a long time, but eventually their cells stop dividing and turn into parenchyma (Evert, 2006; Beck, 2010).

Peripheral (flank) meristems

This type is often considered as part of the shoot apical meristem. The peripheral zone (or initial ring, anneau initial in French authors) is located on the flanks of the apical dome, directly under the tunica. Cells here divide particularly actively and give rise to leaf primordia and axillary meristems — the rudiments of lateral shoots. Thus, the peripheral meristem is the main organogenic centre of the shoot (Serebryakova et al., 2006; Evert, 2006).

Wound (traumatic) meristems

Wound meristems arise at sites of tissue damage from living parenchyma cells that dedifferentiate (return to a meristematic state). Their division leads to the formation of callus — a mass of unspecialised cells, which can then differentiate into cork (periderm) or, in some cases, into adventitious buds or roots. Wound meristems play a key role in plant regeneration, which is widely used in horticulture (grafting, cuttings) and biotechnology (callus culture) (Evert, 2006; Beck, 2010).

Brief summary table

For clarity, the main types of meristems by position are summarised in the table.

Table. Classification of meristems by position in the plant.

Meristem type	Localisation	Primary function	Examples
Apical	Tips of shoots and roots	Primary growth (elongation), initiation of leaves and flowers	Shoot cone, root apical meristem
Lateral	Cylindrical layers in stems and roots (cambium, phellogen)	Secondary growth (thickening), periderm formation	Vascular cambium, cork cambium (phellogen)
Intercalary	Inserted regions in internodes (grasses, horsetails), at leaf bases	Rapid elongation (of stem, leaf)	Wheat internodes, field horsetail
Peripheral	Flanks of the shoot apical meristem (initial ring)	Initiation of leaf and flower primordia, axillary buds	Flank zone of the growth cone
Wound	Sites of injury	Regeneration, callus formation, wound healing	Callus on a cutting, graft union callus

This classification by position allows one to quickly determine which type of growth (primary or secondary) predominates in a given part of the plant and which meristems are responsible for the formation of specific organs.

3.3. Classification by activity

According to the degree and nature of activity, meristems are subdivided into obligate (permanent) and facultative (temporary). This division reflects their role in growth and ability for long‑term self‑maintenance.

Obligate (permanent) meristems are tissues that never fully differentiate and retain the ability to divide throughout the life of the plant. They contain their own population of initial cells, which ensure self‑renewal of the meristem. Obligate meristems include apical meristems (shoot and root) and lateral meristems (cambium and phellogen). They are responsible for indeterminate growth (Evert, 2006; Beck, 2010).

Facultative (temporary) meristems are regions of meristematic tissue that arise from differentiated cells temporarily (e.g., during initiation of leaf primordia, axillary buds, intercalary growth) and eventually fully differentiate, losing the ability to divide. They do not have their own permanent initials. These include:

Intercalary meristems of grasses and horsetails (by the end of the growing season, their cells turn into mature parenchyma).
Peripheral (flank) meristems — their activity is associated with the formation of specific primordia, after which they cease to function as meristems.

Dormant buds (e.g., in trees) are axillary meristems that may remain dormant for a long time, but upon activation (after removal of the terminal bud) they awaken and give rise to lateral shoots. They can also be considered facultative.

Wound meristems — arise only in response to injury and usually differentiate after performing their regenerative function (Evert, 2006; Serebryakova et al., 2006).

Understanding this division is important for practical agronomy: for example, pinching (removal of the apical meristem) activates dormant axillary meristems, leading to tillering and increased yield.

4. Processes: how the meristem works

A meristem is not just a group of dividing cells, but a dynamic system in which several fundamental processes proceed in a coordinated manner: the cell cycle (with mitosis), growth by expansion, and cytodifferentiation. These processes are spatially and temporally separated within the meristem, forming a gradient from dividing initials through the elongation zone to mature tissues.

4.1. Cell cycle and mitosis in the meristem

The basis of any meristem’s operation is the cell cycle — the period of a cell’s existence from one division to the next. In meristematic cells it is significantly shorter than in differentiated ones, ranging from several hours to several days depending on the plant species, meristem type and environmental conditions (Evert, 2006; Beck, 2010).

The cell cycle is classically divided into four phases:

G1 (presynthetic phase, gap 1) — a period of active cell growth after mitosis, synthesis of proteins, RNA and preparation for DNA replication. In this phase, cells of the quiescent centre can remain arrested for a long time, retaining the potential to divide.
S (synthetic phase, synthesis) — DNA replication occurs, doubling the chromosomal material. This is a key stage after which the cell is genetically ready for division.
G2 (postsynthetic phase, gap 2) — synthesis of proteins (including tubulin for the spindle), RNA continues, and preparation for mitosis is completed.
M (mitosis) — actual nuclear division, usually followed by cytokinesis (division of the cytoplasm and formation of a new cell wall). Mitosis in meristems proceeds according to the general scheme: prophase → metaphase → anaphase → telophase (Serebryakova et al., 2006).

Features of mitosis in plant meristems:

Absence of centrioles (in higher plants); the spindle is formed by microtubules organised by specialised zones — poles.
Formation of a phragmoplast in telophase — a structure guiding the deposition of new cell walls between daughter nuclei. Cell division is always centrifugal — from the centre to the periphery.
In lateral meristems (cambium), periclinal divisions (parallel to the surface) predominate, increasing the number of cell layers and organ thickness, whereas in the tunica of the apical meristem, anticlinal divisions (perpendicular to the surface) dominate, increasing the surface area.

Regulation of the cell cycle in meristems is carried out by a complex network of factors: phytohormones (auxins, cytokinins, gibberellins), peptide signals (e.g., CLE peptides), regulatory proteins (cyclins, cyclin‑dependent kinases) and the state of the quiescent centre. Cytokinins stimulate cell division, while auxins are important for maintaining meristematic identity (Aichinger et al., 2012; Fischer et al., 2019).

Spatial organisation of the cell cycle within a meristem is heterogeneous:

In initial cells (including those of the quiescent centre), mitotic activity is minimal or absent — they divide rarely, maintaining the stem cell pool.
The highest mitotic activity is observed in the zone immediately adjacent to the initials (subinitial zone), where cells cycle rapidly, giving rise to a mass of derivatives.
In the peripheral zones of the meristem (peripheral zone of the shoot), mitotic activity is also high, but directed towards the initiation of primordia.

Understanding the cell cycle in meristems allows us to manipulate plant growth: for example, treatment with cytokinins stimulates cambial cell division and increases productivity, while retardants (gibberellin inhibitors) slow mitotic activity, which is useful for compact forms (Evert, 2006).

4.2. Growth by expansion (cell expansion)

After a cell produced in the meristem stops dividing, it enters the phase of growth by expansion (or cell expansion). This is a process of irreversible increase in cell size, which can increase its volume tens or even hundreds of times compared to the original meristematic state. It is growth by expansion that provides the bulk of organ enlargement (e.g., elongation of stem or root, expansion of the leaf blade), whereas cell division merely supplies the “building material” (Beck, 2010; Evert, 2006).

Mechanism of growth by expansion

Three interrelated factors play a key role in this process: turgor pressure, changes in cell wall properties (the wall becomes plastic), and water potential.

Formation of the central vacuole. A young cell, recently detached from the meristem, contains many small vacuoles. During the expansion phase, they fuse to form one large central vacuole, which can occupy up to 90% of the cell volume. The vacuole accumulates osmotically active substances (sugars, salts, organic acids), significantly lowering the water potential inside the cell (Serebryakova et al., 2006).
Water uptake and turgor. Water enters the cell osmotically from neighbouring cells or the apoplast. The growing pressure of the protoplast on the cell wall (turgor) tends to stretch it.
Cell wall loosening (relaxation). For the wall to stretch under internal pressure, it must become plastic. This process is regulated by a complex of factors:
- “Acid growth”: auxin (IAA) activates proton pumps (H^\+-ATPases) in the plasma membrane, acidifying the apoplast (pH drops to 4.5–5.0). Low pH activates expansin enzymes, which break hydrogen bonds between cellulose microfibrils and matrix polysaccharides, making the wall more amenable to stretching (Beck, 2010; Evert, 2006).
- Wall remodelling enzymes: xyloglucan endotransglycosylases (XET) and other proteins rearrange the polysaccharide network, allowing microfibrils to slide relative to one another.
Synthesis of new wall. Simultaneously with expansion, synthesis of new wall components (cellulose, hemicelluloses, pectins) occurs to prevent thinning and rupture. Material is delivered by Golgi vesicles and integrated into the expanding network (Evert, 2006).

Orientation of expansion: role of microtubules and cellulose

A key observation: cells grow predominantly in a direction perpendicular to the orientation of cellulose microfibrils in the wall. Microfibrils act as “reinforcement”, restricting expansion in one direction and directing it in another.

If microfibrils are randomly arranged, the cell grows uniformly in all directions (isodiametric expansion).
If microfibrils are oriented transversely (as rings around the cell axis), the cell grows predominantly in the longitudinal direction (becoming elongated). This orientation is created by cortical microtubules, which guide the movement of cellulose‑synthesising complexes (rosettes) in the plasma membrane (Beck, 2010; Evert, 2006).

In the elongation zone, divisions cease, the phragmoplast disappears, and microtubules reorient, setting the vector for the final cell shape. Hormonal control (auxins, gibberellins) acts precisely at this stage, stimulating wall loosening and microtubule reorientation.

4.3. Cytodifferentiation

Cytodifferentiation (or cell differentiation) is the process by which a cell that has left the meristem gradually acquires structural and functional features characteristic of a particular type of permanent tissue (vessels, sieve tubes, fibres, parenchyma, etc.). Differentiation is the realisation of a genetic programme under the influence of positional information and interactions with neighbouring cells (Evert, 2006; Serebryakova et al., 2006).

Key principles of cytodifferentiation

Positional control, not linear descent. The decisive factor is not which particular initial the cell came from, but its final position in the developing organ. Laser ablation experiments (in Arabidopsis thaliana) have shown that destroyed cells can be replaced by neighbours, which then differentiate according to their new position (Dolan et al., 1993; Evert, 2006). This is a fundamental difference from animals, where cell fate is often determined by a strictly fixed lineage.
Determination and competence. Determination is the fixation of a developmental pathway (the cell already “knows” what it will become, although it may still look unspecialised). Competence is the ability of a cell to respond to a differentiating signal. For example, only cells located in a particular position relative to the auxin flow are competent to differentiate into vascular elements (Evert, 2006).
Gradualness and gradient. Differentiation occurs gradually as the cell moves away from the meristem. In the root, a gradient can be observed: near the tip — dividing cells; above — growing cells; further above — beginning to differentiate. Phloem usually differentiates first, then xylem, and finally storage tissues (Evert, 2006).

Main cytological changes during differentiation

Depending on the final function of the cell, differentiation involves different sets of changes:

Development of the vacuolar system: small vacuoles fuse into one large central vacuole (characteristic of most parenchyma cells). In mature cells, it provides storage and osmoregulatory functions.
Transformation of plastids: proplastids turn into chloroplasts (in photosynthetic tissues), leucoplasts (in storage tissues, e.g., amyloplasts for starch) or chromoplasts (in petals and fruits to attract pollinators/dispersers) (Serebryakova et al., 2006).
Thickening and modification of the cell wall: synthesis of a secondary cell wall, deposition of lignin (in xylem and sclerenchyma), suberin (in cork, endodermis) or cutin (in the epidermis). These changes are irreversible and often lead to protoplast death (e.g., in vessels, tracheids, fibres).
Specialised changes in the protoplast:
- In sieve tubes of the phloem, the nucleus and most organelles degenerate, the cytoplasm remains only in a parietal layer, phloem protein (P‑protein) and callose appear on sieve plates (Evert, 2006).
- In glandular and secretory cells, the endoplasmic reticulum and Golgi apparatus are highly developed for the synthesis of essential oils, nectar, resins.
- In companion cells of sieve tubes, dense cytoplasm with a large nucleus and numerous mitochondria is retained to support the metabolism of neighbouring sieve elements.
Programmed cell death (PCD): In tracheary elements (vessels and tracheids) and fibres, differentiation ends with PCD, in which the protoplast is lysed, while the walls remain and perform conducting or supporting functions (Evert, 2006).

Differentiation and totipotency

It is important to emphasise that even differentiated cells retain totipotency — the ability, under certain conditions (e.g., in tissue culture, upon wounding), to “remember” their meristematic past, dedifferentiate and give rise to a new plant. This property is actively used in plant biotechnology (clonal micropropagation, production of haploids from pollen grains) (Evert, 2006; Beck, 2010).

Thus, the processes of the cell cycle, growth by expansion, and differentiation form a single continuum of transformations that turn a meristematic cell from a “universal building block” into a specialised element of plant tissue. Understanding these processes is the key to managing the growth, development and productivity of crop plants.

4.4. The three zones of a meristem

The operation of a meristem is not limited to the zone of dividing cells. In a typical apical meristem (especially the root), three successive zones can be distinguished, which reflect different stages of cell development: the division zone, the elongation (growth) zone, and the differentiation zone. These zones gradually merge into one another, creating a gradient from unspecialised dividing cells to mature tissues (Evert, 2006; Beck, 2010).

Division zone (meristematic zone)

This is the most distal (in the root — closest to the tip) part of the meristem, where cells maintain high mitotic activity. In the root, it is protected by the root cap and occupies the first 250–500 μm from the tip (in Arabidopsis about 250 μm). Characteristic features:

Cells are small, isodiametric, with dense cytoplasm and small vacuoles.
High mitotic index (percentage of cells in mitosis).
Presence of a quiescent centre (in the root) — a group of cells in the centre of the division zone with low mitotic activity, serving as a reserve for meristem restoration (Dolan et al., 1993; Dubrovsky & Vissenberg, 2021).
In the shoot, the division zone includes the central zone (corpus) and tunica, as well as the flank meristems (peripheral zone) where leaf primordia are initiated (Evert, 2006; Aichinger et al., 2012).

Elongation zone

Located proximal to the division zone (closer to the base of the organ). Here, cells stop dividing but actively grow by expansion. This zone can reach several millimetres (in the Arabidopsis root — about 250–500 μm). Features:

A large central vacuole forms, cells increase in length (in the root — up to 10–20 times compared to meristematic cells).
Cell walls undergo plastic deformation (under turgor and enzymatic action) and new layers are simultaneously deposited.
Microtubules reorient transversely, directing cellulose deposition, which determines the growth vector (usually longitudinal) (Beck, 2010).
Intensive synthesis of protein and RNA, aimed at building new cytoplasmic and wall components.
In this zone no divisions occur, and the cells are not yet fully differentiated (no specialised wall thickenings or specific inclusions).

Differentiation zone (maturation zone)

The most proximal zone, where cells acquire their final structural and functional features. It begins where active elongation ends. Here:

Cells complete differentiation: secondary walls form (vessels, fibres), suberin or lignin are deposited, specialised inclusions appear (starch in amyloplasts, proteins, anthocyanins).
Programmed cell death occurs for xylem conducting elements and some mechanical tissues.
In the root, in this zone, root hairs form from trichoblasts, and the endodermis acquires Casparian strips (Evert, 2006; Serebryakova et al., 2006).

Important: the boundaries between zones are not sharp. For example, in the root, protophloem (the first sieve tubes) may begin to differentiate already in the elongation zone, and some parenchyma cells continue to expand even after differentiation has started (Evert, 2006).

4.5. Programmed cell death as part of differentiation

Programmed cell death (PCD) is a genetically controlled process of active self‑destruction of a cell, which is an integral part of normal development in many plant tissues. In the context of meristems and their derivatives, PCD is the logical conclusion of differentiation for those cells that, in the mature state, must be dead yet continue to perform their functions (Evert, 2006; Beck, 2010).

Classic examples of PCD in plant tissues

Differentiation of tracheary elements (vessels and tracheids) of xylem. This is the most studied example. The precursor cell (vessel element) first deposits a secondary lignified wall (annular, spiral, reticulate or pitted). Then, after wall completion, protoplast lysis occurs: the vacuole ruptures, releasing hydrolytic enzymes (proteases, nucleases) that break down the nucleus, cytoplasm and all organelles. The plasma membrane is preserved briefly, but eventually also breaks down in the region of perforation plates, creating a continuous water‑conducting lumen. Only the cell wall remains (sometimes with plasma membrane remnants). Key regulators of PCD in xylem are VND6 and VND7 proteins (Fischer et al., 2019; Evert, 2006).
Formation of sclerenchyma fibres. Similarly, fibres (bast and wood fibres) die after depositing thick secondary walls, performing a supporting function. Their lumen is filled with air or, rarely, with protoplast remnants.
Aerenchyma. In aquatic and wetland plants, in response to hypoxia (oxygen deficiency), root cortex cells initiate PCD, resulting in large air‑filled spaces (aerenchyma) that facilitate gas exchange. This process is induced by ethylene and involves activation of hydrolases (Evert, 2006).
Root cap cells. The outer cells of the root cap are continuously sloughed off and die, replaced by new ones formed from the calyptrogen. This is an example of PCD in a constantly renewing tissue.
Sensory cells during leaf mosaic formation (perforated leaves). In some plants (e.g., Monstera), patches of leaf blade tissue die in a programmed manner, creating characteristic holes.
Completion of sieve tube development in phloem. Although sieve elements remain alive, they also undergo “simplified” PCD: degradation of the nucleus, disappearance of the tonoplast and most organelles, but retention of the plasma membrane, some plastids and smooth ER. This is not complete death, but selective autolysis, allowing the cell to retain the ability to transport assimilates (Evert, 2006).

Molecular mechanisms and features of PCD in plants

PCD in plants differs from apoptosis in animals, although there are some common features.

Apoptosis in animals: accompanied by cell shrinkage, nuclear fragmentation, formation of apoptotic bodies and phagocytosis. Not typical for plants due to the presence of a rigid cell wall (Evert, 2006).
PCD in plants: most often associated with vacuole rupture and release of hydrolases that lyse the protoplast from within. Morphological signs: chromatin condensation, DNA fragmentation (ladder pattern), nuclear rounding, nucleolus disappearance. The vacuole acts as a “lysosomal compartment”.

Regulation of PCD is carried out by complex signalling cascades: hormones (ethylene, abscisic acid, gibberellins), reactive oxygen species (ROS), proteins (Rac/Rop GTPases), transcription factors (NAC‑domain proteins). For example, in Arabidopsis the VND6 and VND7 proteins directly activate genes encoding hydrolases, as well as genes involved in secondary wall synthesis (Fischer et al., 2019; Evert, 2006).

Significance of PCD for meristems and development

Programmed cell death is not a catastrophe but a tool for morphogenesis. In the context of meristems, it:

Makes room for new tissues (e.g., during aerenchyma cavity formation).
Creates functional conducting elements (vessels, tracheids).
Participates in the removal of obsolete structures (e.g., root cap cells).
Protects the plant from pathogens through the hypersensitive response — rapid PCD of cells around the infection site, isolating the infection (Evert, 2006).

Thus, PCD is the final act in the development of specialised tissues derived from meristems. Understanding the mechanisms of PCD opens up opportunities for managing wood quality (e.g., reducing lignification for the pulp and paper industry) and plant stress resistance.

5. Factors regulating meristem activity

Meristem activity is neither autonomous nor constant. It is finely regulated by a complex network of endogenous signals (phytohormones, peptides) and modulated by external conditions (light, temperature, photoperiod, mechanical stimuli). This allows the plant to adapt its growth and development to a changing environment. Understanding these regulators is key to managing crop yield, wood quality and stress tolerance.

5.1. Hormonal regulation

Phytohormones act as the main chemical messengers controlling cell division, expansion and differentiation in meristems.

Auxins (IAA, indole‑3‑acetic acid)

Auxins are central regulators of meristematic activity.

Sources: Shoot apical meristems, young leaves, developing seeds. The main natural auxin is IAA.

Polar transport: Movement of auxin from the apex to basal parts (polar transport) creates concentration gradients that act as positional signals. Transport is carried out by specific carrier proteins: AUX/LAX (influx) and PIN (efflux) (Fischer et al., 2019; Evert, 2006).

Effects on meristems:

Shoot apical meristem: Auxin synthesised in young leaves and transported to the base of primordia induces new leaf initiation and suppresses the outgrowth of axillary buds (apical dominance). High auxin concentrations in the central zone maintain stem cell identity (Aichinger et al., 2012).
Cambium: The auxin maximum in the cambial zone stimulates cambial cell division and the differentiation of derivatives into xylem (towards high concentrations) and phloem (towards low concentrations). Removal of the terminal bud (the auxin source) stops cambial activity; exogenous auxin restores it (Fischer et al., 2019).
Root apical meristem: Auxin accumulates in the quiescent centre (QC), maintaining its organising function. A local auxin maximum determines the position of the QC and regulates initial cell division (Dubrovsky & Vissenberg, 2021).

Cytokinins

Cytokinins act as antagonists of auxins in many processes and stimulate cell division.

Synthesis: Primarily in roots (especially in the root apical meristem), then transported to the shoot via xylem.

Effects on meristems:

Cambium: Cytokinins stimulate cambial cell division and increase phloem production. Together with auxins they maintain the balance between xylem and phloem (Fischer et al., 2019).
Shoot apical meristem: Cytokinins produced in roots activate cell division in the meristem, promote the release of axillary buds from dormancy (overcoming apical dominance) and increase meristem size.
Quiescent centre (QC): Cytokinins, together with auxins, are necessary for maintaining QC cell quiescence and stem cell identity (Aichinger et al., 2012).

Gibberellins (GAs)

Gibberellins are stimulators of growth by expansion and, in certain contexts, of division.

Effects: Elongation of internodes by activating intercalary meristems (grasses), stimulation of cambial cell division and, importantly, differentiation of xylem fibres (increase fibre length and number in wood). Application of gibberellins is used to enhance growth and modify wood quality (Fischer et al., 2019; Evert, 2006).

Ethylene

A gaseous hormone regulating many developmental processes and stress responses.

Effects: Stimulates aerenchyma formation via PCD, can activate cambial cell division (especially during formation of tension wood in hardwoods in response to mechanical stress). Involved in fruit ripening and leaf senescence (Evert, 2006; Fischer et al., 2019).

Abscisic acid (ABA)

ABA is a stress and dormancy hormone.

Effects on meristems: High ABA levels induce dormancy of apical meristems at the end of the growing season (especially in perennials). ABA is also involved in stomatal closure and adaptation to drought, indirectly affecting meristem activity through changes in water status (Evert, 2006).

Jasmonates (JA, JAs)

Jasmonates are important regulators of development and defence.

Effects: Involved in cambium formation and secondary growth (especially under mechanical load). Jasmonates also activate defence responses that can modulate meristem activity when damaged by insects or pathogens (Fischer et al., 2019).

5.2. Peptide regulation (CLE peptides)

CLE peptides (CLAVATA3/ESR‑related) are a family of signalling molecules that play a key role in maintaining meristem stem cells.

Mode of action: Secreted into the apoplast, they bind to receptor kinases (e.g., CLV1, CLV2, CRN, BAM) on neighbouring cells, triggering a signalling cascade that suppresses the expression of regulatory genes (e.g., WUSCHEL in the shoot) (Aichinger et al., 2012; Fischer et al., 2019).

Role in the shoot: CLV3, expressed in stem cells (central zone), binds to the CLV1 receptor in underlying cells (the organising centre), limiting the size of the stem cell pool (negative feedback). Mutations in the CLV system lead to giant meristems and fasciation.

Role in the root: CLE40 regulates columella cell differentiation.

Role in the cambium: TDIF (CLE41/44), secreted by phloem cells, binds to the PXY/TDR receptor in the cambium, stimulating cambial cell division and suppressing xylem differentiation.

5.3. External (environmental) factors

Light

Light is a key signal for activating meristems after germination. It acts not only through photosynthesis (energy supply) but also through phytochromes and cryptochromes, which regulate gene expression, including genes involved in cytokinin signalling (Aichinger et al., 2012). Photoperiod controls the transition from vegetative growth to flowering, changing the working pattern of the shoot apical meristem.

Temperature

Temperature significantly affects the speed of the cell cycle in meristems. Moderate warmth (optimum) accelerates division, low temperatures slow or stop it (vernalisation in winter crops requires prolonged exposure to low positive temperatures to induce flowering). Extreme temperatures can damage meristems. In perennials, short photoperiod and decreasing temperature in autumn induce the transition of the apical meristem to a dormant state (dormancy) (Evert, 2006).

Photoperiod

Day length is a crucial signal for seasonal regulation of meristem activity (especially in perennials and winter forms). Short days in autumn induce the formation of dormant buds and cessation of cambial activity in trees. Long days in spring and summer activate growth. In plants that flower at a specific day length, photoperiod determines the switch of the apical meristem from vegetative to reproductive development (Evert, 2006; Aichinger et al., 2012).

Mechanical signals

Mechanical stress (wind, rain, touch, self‑weight) causes thigmomorphogenesis — reduced elongation growth and increased secondary thickening (the stem becomes sturdier). Mechanical stress activates the expression of genes related to auxin, ethylene and jasmonates, and stimulates cambial cell division. This phenomenon is used in the practice of topping (removal of shoot tips) to enhance branching, and in growing wind‑resistant tree plantations (Fischer et al., 2019; Evert, 2006).

5.4. Endogenous regulatory mechanisms: apical dominance

Apical dominance is the suppression of the growth of axillary (lateral) meristems by the shoot apical meristem. The main mechanism: auxin, synthesised in the apical meristem and young leaves, is transported downwards in the stem and inhibits the activation and growth of axillary buds. Removal of the apical meristem (pinching, tipping) removes the auxin source, leading to activation of axillary meristems and branching. This principle is widely used in horticulture and crop production to shape bushes, increase the number of fruiting shoots and enhance yield (Evert, 2006; Serebryakova et al., 2006).

Thus, meristem activity is the result of complex interactions between endogenous and exogenous factors. Understanding these regulatory networks allows the agronomist to deliberately influence the growth and development of crop plants, optimising their architecture, flowering time and productivity.

6. Agronomic and practical significance

Meristems are not only a fundamental object of botanical science but also an essential tool in the hands of agronomists, horticulturists, foresters and biotechnologists. By controlling the activity of meristematic tissues, one can purposefully modify plant architecture, stimulate root formation, regulate flowering and fruiting times, increase lodging resistance, and use vegetative propagation methods to preserve valuable genotypes. Understanding how meristems work underpins many agronomic practices and modern biotechnologies.

6.1. Controlling plant growth and shape (architectural training)

Apical dominance: tip removal (pinching, topping, tipping)

As noted in section 5.4, the shoot apical meristem suppresses the outgrowth of axillary buds via an auxin signal. Artificially removing the apical meristem (pinching, topping, tipping) removes this suppression, leading to the activation of dormant axillary meristems and increased branching.

In horticulture and vegetable growing: Shaping bushes of tomatoes, peppers, cucumbers, eggplants yields a compact plant with more fruiting shoots and increased yield. In fruit trees, crown training (removal of the central leader) creates a shape convenient for maintenance and harvesting.
In ornamental floriculture: Pinching tips stimulates tillering, making the plant more bushy and floriferous (Evert, 2006; Serebryakova et al., 2006).
Topping (in viticulture, etc.): Partial removal of the shoot tip not only enhances branching but also redirects assimilates to the remaining organs (e.g., grape clusters), improving crop quality.

Suckering (removal of lateral meristems)

Suckering is the removal of axillary meristems (suckers), especially at early stages of their development. Unlike pinching, it is the lateral, not the main, meristems that are removed. Used mainly in tomato cultivation: removing suckers (second‑order shoots) redirects nutrients to flower trusses, accelerating fruit ripening and increasing fruit size (Evert, 2006).

Use of retardants

Retardants — chemical compounds (gibberellin inhibitors) that suppress the activity of intercalary meristems and growth by expansion. They do not kill the plant but only slow internode elongation, making the stem shorter and thicker (lodging‑resistant). Widely used in cereal farming (wheat, barley, rice) and in floriculture to obtain compact plants with sturdy flower stalks. Reducing stem height lowers the risk of lodging before harvest, facilitating mechanical harvesting and reducing yield losses (Evert, 2006).

6.2. Stimulation of root formation

The ability of differentiated cells to dedifferentiate and form adventitious roots is widely used in vegetative propagation by cuttings.

Auxins (heteroauxin, rooting powder, IBA): Treating the cut end of a cutting with exogenous auxins stimulates dedifferentiation of cambium, pericycle and parenchyma cells in the wound region, followed by their differentiation into meristematic foci, from which adventitious roots develop. This is especially important for difficult‑to‑root species (conifers, many fruit trees) (Evert, 2006; Serebryakova et al., 2006).
Use in forestry and horticulture: Cuttings with auxin treatment are the primary means of propagating many ornamental and fruit crops, as well as some timber species.

6.3. Vegetative propagation and genotype conservation

The ability of meristems to divide and the totipotency of plant cells underlie virtually all methods of vegetative propagation (cuttings, layering, grafting, tissue culture). This allows the production of genetically identical clones, preserving valuable cultivar traits.

Microclonal propagation (meristem culture): Isolating apical meristems (often 0.1–0.5 mm in size) and cultivating them on nutrient medium yields large numbers of virus‑free regenerated plants. Apical meristems are generally virus‑free (viruses do not invade the division zone), making this method indispensable for sanitising planting material of potato, berry crops, fruit trees, ornamentals and flowers (Evert, 2006).
Callus cultures: Callus (a mass of dedifferentiated cells) can be induced on any explant. From callus one can either directly obtain adventitious shoots or use it for suspension culture and subsequent plant regeneration. This is the basis for somatic hybridisation, genetic engineering, and production of haploid plants (from pollen grains).
Regeneration from dormant buds: In many trees and shrubs (oak, birch, linden), dormant axillary meristems persist in old wood and can be activated upon injury (stump sprouting). This is used in forestry to regenerate forests after cutting (Serebryakova et al., 2006).

6.4. Agroecosystem aspects: links to yield and resilience

Cambium activity → stem thickness → lodging resistance and biomass yield

Vascular cambium activity not only determines long‑term thickening of tree trunks but also, in annual and biennial crops, the stem diameter. A thicker stem (due to robust secondary xylem and mechanical fibres) is more resistant to lodging, especially during grain filling (cereals) and fruiting (tomatoes, sunflower). Moreover, a larger cross‑section of conducting tissues improves water and mineral transport to the apex and assimilate flow from leaves to storage organs (root crops, tubers, grains). Therefore, breeding for enhanced cambium development (without excessive vegetative mass) is an important direction for increasing productivity of cereals, industrial and forage crops (Fischer et al., 2019; Evert, 2006).

Wood quality (forest crops)

In forest tree species (pine, spruce, oak, ash), wood quality directly depends on cambium activity. Regulating the ratio of earlywood to latewood, tracheid and fibre length, and cell wall thickness — all are determined by cambial meristem activity and the differentiation of its derivatives. Understanding hormonal (auxins, gibberellins, jasmonates) and environmental regulation of the cambium enables breeding for improved wood mechanical properties, density and decay resistance (Fischer et al., 2019).

Recovery after damage

Meristems (especially wound and dormant meristems) play a key role in plant recovery after biotic and abiotic stresses: freezing, hail damage, pest attack. Activation of dormant buds and formation of adventitious shoots from the pericycle meristem (root suckers) allow the plant to regenerate and survive extreme conditions. This must be taken into account in plant protection strategies and when establishing perennial plantations (Serebryakova et al., 2006).

References

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Beck, C. B. (2010). 'An Introduction to Plant Structure and Development: Plant Anatomy for the Twenty-First Century', 2nd ed. Cambridge University Press. (Chapter 5: Meristems of the shoot and their role in plant growth and development, pp. 77-104)
Dolan, L., Janmaat, K., Willemsen, V., Linstead, P., Poethig, S., Roberts, K., & Scheres, B. (1993). 'Cellular organisation of the Arabidopsis thaliana root'. Development, 119(1), 71-84.
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