Plant Dermal Tissues

Origin: from the protoderm of the shoot apical meristem.

Localization: covers all primary shoot organs (leaves, stems, flowers, fruits), as well as seeds.

Characteristics: usually uniseriate, composed of tightly packed living cells (parenchymatous type), lacks intercellular spaces (except for stomata). The outer cell walls and cuticle are impregnated with cutin and waxes (Evert, 2006).

Derivatives of the epidermis:

Origin: from the outer cells of the root apical meristem, often with involvement of the root cap (calyptrogen).

Localization: covers young roots in the absorption zone.

Characteristics: uniseriate, living cells, lacking a cuticle, thin-walled, which ensures absorption of water and mineral salts. In most plants, rhizodermis cells form root hairs — unicellular outgrowths that increase the absorbing surface area (Evert, 2006; Beck, 2010).

Origin: a specialized form of multilayered epidermis (derived from protoderm).

Localization: on aerial roots of epiphytes (e.g., orchids, Orchidaceae; some aroids, Araceae).

Characteristics: multilayered (up to 10–20 layers), velamen cells are dead, with porous spiral or reticulate wall thickenings. Velamen acts as a capillary sponge: it quickly absorbs and retains rainwater or condensate, creating a water reserve for the living cells of the cortex (Beck, 2010; Crang et al., 2018).

A collective term denoting a three-layered complex that replaces the epidermis and performs a protective function.

Cork (phellem):

Cork cambium (phellogen):

Phelloderm:

Origin: formed by repeated formation of successive periderms deep within the cortex (phloem or primary cortex).

Localization: on old tree trunks and perennial branches (e.g., in Quercus, Pinus, Platanus).

Characteristics: a thick, massive, often fissured complex consisting of alternating layers of dead cork and enclosed dead cortical (phloem) tissues (Evert, 2006). Individual periderms in the rhytidome may be not continuous but scale-like (scaly bark) or, more rarely, concentric (ring bark).

Function: maximal protection against mechanical damage, ground fires, sharp temperature fluctuations and biological damage.

The epidermis is established from the protoderm — the outer layer of cells of the shoot apical meristem (growing tip). Even before active growth of the leaf or stem begins, protoderm cells on the surface acquire characteristics that distinguish them from internal cells (initials): they become flatter, and their vacuolization slows down (Evert, 2006). Later, as the organ elongates, these cells finally differentiate into typical epidermal cells with sinuous anticlinal walls, cuticle and waxy bloom.

Importantly, the epidermis contains meristemoids — specialized cells that retain the ability to divide and give rise to specialized structures:

The epiblema is formed from the outer cells of the root apical meristem, which already in the division and elongation zone become distinguished from internal tissues. Its key feature is the formation of root hairs. A root hair is an outgrowth of a trichoblast (a specialized epiblema cell) resulting from local softening and protrusion of the cell wall. Root hair growth occurs exclusively by tip growth, similar to pollen tube growth, and depends on a Ca²⁺ gradient and actin cytoskeleton activity (Evert, 2006). The life of a single hair is short (a few days), but it is constantly replaced by new hairs from younger root regions.

When the stem or root begins to thicken due to cambial activity, the epidermis can no longer stretch and protect the growing volume. Then, beneath the epidermis or in deeper layers of the cortex (primary cortex or even the phloem), dedifferentiation of parenchyma cells occurs. They lose their vacuoles, condense their cytoplasm, and begin to divide actively, turning into phellogen — a uniseriate lateral meristem (Evert, 2006; Beck, 2010).

In different species, phellogen may arise:

Phellogen cells divide periclinally (i.e., with the division plane parallel to the organ surface), leading to an increase in cell number in the radial direction:

Because the stem continues to grow in thickness, the phellogen must stretch. This is achieved by anticlinal divisions (divisions in the radial plane, perpendicular to the surface) — they increase the number of initial cells around the circumference (Evert, 2006). As a result, the periderm maintains its integrity.

In most trees, after a few years the first periderm cracks and no longer protects the enlarging trunk. Then a new periderm is formed in deeper cortical layers (sometimes in the phloem). The process repeats many times: each successive periderm cuts off from the internal tissues those parts that end up outside it and die. This forms the rhytidome — a complex of alternating layers of cork and dead cortical tissue (Flämmig, 2020). For example, in cork oak (Quercus suber), the phellogen can remain active for decades, producing a thick layer of cork that is then harvested for industrial use (Faustino et al., 2023).

The epidermis covers all primary shoot organs: leaves, herbaceous stems, flowers, fruits, and seeds. In perennial woody plants, the epidermis is sloughed off with the appearance of the periderm, but on young growth and leaves it remains throughout the growing season (Evert, 2006).

The epidermis is a complex tissue comprising several cell types:

Cuticle. This is a continuous extracellular film covering the outer surface of the epidermis. It consists of two main components:

Functions of the cuticle: reduction of uncontrolled transpiration (main barrier), reflection of UV rays, imparting hydrophobicity (“lotus effect”), protection against pathogens. Cuticle structure and thickness depend on ecology: in xerophytes it is thick, in hydrophytes it is very thin or absent (Evert, 2006).

Pectin layer. Between the cuticle and the cell wall proper there is often a thin layer of pectins that binds the cuticle to the primary wall. Pectins help maintain cuticle integrity and may affect its permeability (Yeats & Rose, 2013).

Cellular composition: a single layer of living parenchyma cells. Cuticle absent — this is the main difference from the shoot epidermis. Cells are thin-walled, necessary for water and ion absorption.

Root hairs: outgrowths of trichoblasts — specialized epiblema cells (all cells may be trichoblasts, or only some, depending on the species). A root hair is unicellular, 0.1 to 8 mm long, lives for a few days. Its wall consists of cellulose and pectins, the outer surface is often covered with mucilage, facilitating contact with soil particles and mycorrhizal fungi (Evert, 2006; Beck, 2010).

A multilayered (up to 10–20 layers) epidermis, whose cells in the mature state are dead; their walls have spiral or reticulate thickenings (cellulosic and lignified), as well as pores.

When dry, the velamen is white (light-reflecting); when wet, it becomes translucent. The capillary system between dead cells quickly absorbs and retains water (rainwater, condensate), which then passes through specialized passage cells (via the exodermis) into the cortex (Crang et al., 2018).

It consists of three components (Evert, 2006; Faustino et al., 2023):

These are regions of the periderm where the phellogen produces not compact cork but filling tissue — rounded, loosely arranged cells with intercellular spaces.

Lenticels enable gas exchange between internal tissues (living phloem, cortex) and the atmosphere through the impermeable cork layer.

Lenticel shapes vary: round, elongated (along the stem axis), sometimes wart-like. In birch, lenticels appear as transverse dashes on the white bark (Evert, 2006).

Arises when successive periderms are repeatedly formed deep within the cortex. Each new periderm cuts off layers of outer tissues that then die.

The result is a massive complex of alternating layers of cork and dead phloem/cortex (Beck, 2010).

Rhytidome structure determines the appearance of bark: scaly (in pine, oak), fibrous (in juniper), ringed (in grapevine). The rhytidome does not stretch, so as the trunk thickens it cracks, forming deep longitudinal furrows.

Guard cells are surrounded by neighboring epidermal (sometimes subsidiary) cells. When turgor in guard cells increases, their thin dorsal (outer) walls stretch, while the thicker ventral walls (facing the pore) bend, pushing the stomatal pore open (Evert, 2006). When turgor falls, guard cells collapse and the pore closes.

The main osmotically active ion in guard cells is potassium (K^\+). During the light (daytime) period:

In darkness or under stress, the process reverses: K^\+ exits guard cells through other (depolarization-activated) channels, water leaves the cells, turgor falls — stomata close.

Abscisic acid (ABA) is a key stress hormone, especially under water deficit. During drought, ABA accumulates in leaves (synthesized in the mesophyll or roots and transported via the xylem). ABA binds to receptors on the guard cell membrane, triggering a cascade of signaling events:

An increase in internal leaf CO₂ concentration (e.g., on a hot afternoon when photosynthesis is slowed) stimulates stomatal closure. A decrease in CO₂ (active photosynthesis) promotes opening. Blue light photoreceptors (phototropins) trigger a signaling pathway leading to H⁺-ATPase activation and K^\+ uptake, independently of red light (which acts through photosynthesis and CO₂ reduction) (Pillitteri & Torii, 2012).

Signals for phellogen formation: attainment of a certain organ thickness, mechanical stretching, tissue aging (Evert, 2006).

Initial phellogen cells divide periclinally. Outer daughter cells differentiate into phellem (cork), inner ones into phelloderm (living parenchyma) (Faustino et al., 2023; Beck, 2010).

A key step in cork formation is the synthesis and deposition of suberin. Suberin is a biopolymer similar to cutin but with a higher content of unsaturated long-chain fatty acids (C18–C24) and includes both an aliphatic and a phenolic (often ferulic acid) domain (Faustino et al., 2023).

When the bark is damaged (mechanically, by burning, or pathogen invasion), cells around the wound rapidly activate suberin synthesis and form wound periderm. Suberin then forms a so‑called boundary layer that isolates healthy tissues from dead ones and prevents pathogen entry (Chano et al., 2015). This process is controlled by ethylene and jasmonates.

Thus, physiological processes in dermal tissues provide active regulation of gas exchange (stomata), dynamic protection against desiccation (cuticle), and repeated renewal of the outer covering (periderm). Next, we will examine which external factors affect the state of these tissues and how the plant adapts to changing conditions.

Air humidity is one of the main factors determining transpiration intensity. When relative humidity decreases, the leaf loses more water and stomata close (Farquhar & Sharkey, 1982). Prolonged exposure to dry air triggers deeper changes:

High-energy UV-B (280–315 nm) damages DNA and proteins. Dermal tissues act as the first filter:

High temperature. When a leaf overheats, stomata close (due to increased ABA and CO₂), but if the temperature exceeds 40–45 °C, guard cells may be irreversibly damaged. In heat-tolerant plants, the cuticle is thick and epicuticular waxes have a high melting point, preventing them from spreading (Riederer & Schreiber, 2001).

Low temperature (freezing). In perennials, preparation for winter occurs: suberin accumulates in the periderm, cork cells die and serve as thermal insulation. In conifers, the needle epidermis is covered with a thick wax layer, and stomata are plugged with wax, preventing freezing and desiccation in winter (Beck, 2010; Evert, 2006).

Plants from saline habitats (Salicornia, Atriplex) possess specialized glandular trichomes — salt glands. These cells actively accumulate Na^\+ and Cl^- ions and then secrete them onto the epidermal surface as a concentrated salt solution, which crystallizes. This prevents toxic salt accumulation in the tissues (Wagner, 1991; Evert, 2006).

Induction of cuticle and suberin synthesis. Pathogen penetration through the epidermis causes local cuticle thickening and suberin deposition in cells adjacent to the infection site, forming a barrier (Yeats & Rose, 2013).

Secretion of antimicrobial compounds. Glandular trichomes of many plants (e.g., in Nicotiana tabacum) secrete resins and terpenes that have fungicidal and bactericidal activity (Wagner, 1991; Dong et al., 2023).

Stomatal closure. Bacteria (e.g., Pseudomonas syringae) secrete coronatine, which forces stomata to open for entry. In response, plants recognize pathogen‑associated molecular patterns (MAMPs) and close their stomata (Hetherington & Woodward, 2003).

Mechanical defense: covering trichomes create a physical barrier; spines and stinging hairs damage insect mouthparts. In Urtica dioica, trichomes contain histamine and formic acid (Wagner, 1991).

Chemical defense: glandular trichomes secrete toxic or repellent substances (pyrethroids, alkaloids, cannabinoids) (Dong et al., 2023). For example, in Cannabis sativa, trichomes are rich in THC, which repels insects.

Induction of increased trichome density. Insect damage to leaves stimulates the formation of new trichomes on young leaves (systemic acquired resistance), an example of induced defense (Dong et al., 2023).