# Isidium

An isidium (plural: isidia) is a tiny, wart- or finger-like projecting from the thallus surface of certain lichen species. It mainly helps the lichen reproduce. Each isidium contains both fungal and algal partners and is wrapped in a thin protective layer (the cortex), distinguishing it from soredia, which lack this protective covering. Isidia also help lichens colonize new habitats; however, their relatively heavier structure limits their ability to disperse over long distances. Isidia are morphologically diverse, ranging from spherical and cylindrical to club-shaped or scale-like, typically measuring 0.01–0.03 mm in diameter and 0.5–3.0 mm in height, and may be smooth, knobby, shiny, matt, or hollow.
Morphological characteristics of isidia hold significant taxonomic value, influencing species classification and nomenclature in lichenology. Certain specialized forms, such as schizidia and thlasidia, reflect subtle developmental distinctions. Ecologically, isidia significantly increase the thallus surface area, enhancing its ability to retain moisture, exchange gases, and conduct photosynthesis. They disperse primarily via passive vectors such as wind, water, and animals, though their heavier, corticate structure restricts their potential for long-distance dispersal compared to lighter propagules like soredia.
Lichenologists have recognized the importance of isidia for over two centuries, dating back to Erik Acharius in 1794. Features of isidia are reflected in the species epithets of many lichens, both indirectly, and explicitly. Research has since expanded to explore their roles in ecological restoration, including lichen transplantation, as well as their contributions to ecosystem functions and symbiont dispersal networks. Modern molecular approaches have also begun investigating the genetic basis of isidium formation and its evolutionary significance.

## Morphology

Isidia appear in various shapes, including spherical (globose), cylindrical, scale-like (squamulose, coralloid, club-shaped (claviform), disc-shaped (disciform), cup-shaped (cupuliform), and wart-like verruciform. Although they are always small (around 0.01–0.03 mm wide and up to 3 mm tall), their surface may be smooth or knobby, shiny or matt. Most isidia are solid, but some lichens feature hollow, inflated isidia. Soralia and isidia formation can overlap, with soralia growing at the tips of isidia and isidia developing within a soralium, where the loose network becomes compact and forms a secondary cortex.
The isidia of the foliose lichen Pseudocyphellaria horridula are distinct, emerging vertically from the lobes and growing up to 10 mm. These large isidia create a distinctive fish bone pattern on the lobes. Parmelina pastillifera also has distinctive isidia. This greyish foliose species has a central part with button-shaped, brown to black protuberances that have a flat or slightly concave, warty surface. These protuberances break off easily, leaving crater-shaped depressions, distinguishing P. pastillifera from P. tiliacea, which has smaller, globose to cylindrical black isidia on its older, central parts.
Distinguishing isidia from other small bumps or outgrowths, such as warts or papillae, can be difficult. Older isidia sometimes resemble tiny lobes (lobuli) or develop into hair-like strands. Some lichens transform their isidia into new structures without allowing them to detach, though only a few species actively release isidia by weakening the base. The isidia of many crustose lichens, like Pertusariaceae, detach easily, while others remain attached until the thallus dies. In many gelatinous and foliose lichens, the isidia stay attached to the thallus permanently, likely increasing the surface area.
Consoredia are clusters of incompletely separated soredia; soralia with many consoredia may be mistaken for isidia clusters. Sometimes, isidia may break down to soredia and consoredia. Species like Pertusaria coccodes may rarely produce both isidia and soredia. "Sorediate isidia" refers to isidia that erupt into soredia, typically at the tips. "Isidiate (or isidioid) soredia" describes soredia that look like isidia but lack a cortex and arise from distinct soralia. Gustaf Einar Du Rietz used "isidiate soredia" for the fragile isidia of Xanthoparmelia loxodes and X. verruculifera, but the term is inaccurate, as no soredia form. Soredial granules growing like isidia on the parent thallus are common in the family Physciaceae.

## Types and variations

Isidia occur in a range of forms, each offering insights into lichen biology and taxonomy. In his 1992 monograph of the bark-dwelling, sorediate and isidiate crustose lichens of Norway, Tonsberg broadly defined isidia as "all globose to elongate, usually projecting, corticate diaspores with a basal point of attachment", and further defined several types.
Some lichens develop isidia from within the upper layer (cortex). These "burst" through as they grow, pushing older isidia outward. Early in their development, they lack a distinct cortex—this forms later—so internally they are homoiomerous (meaning fungal and algal cells are more evenly mixed). The mature isidia were surrounded by ecorticate tissue in rounded patches, often located at tuberculae tips. This was observed in species like Pertusaria flavida, P. coccodes, and P. coronata.
Some isidia connected continuously with the surrounding thallus cortex and seemed to originate from it. This was seen in species like Pertusaria corallina, P. oculata, and P. dactylina. These isidia were heteromerous, meaning they had a white medullary core (a loosely woven fungal layer) surrounded by an algal layer. This formation mode differs anatomically from the homoiomerous isidia of the foliose species Lasallia pustulata and Parmelia tiliacea, which are also derived from the thalline cortex.
A few areolate species displayed isidiiform areolae, which are more or less spherical to cyclindrical, and easily detached or broken. Examples include Caloplaca herbidella and Placynthiella icmalea. The latter's isidia-like areolae were termed "blastidia", as they resembled consoredia but remained attached at the base to the substrate. The entire areola of Placynthiella icmalea was identified as a blastidium.
An isidiomorph is a structure that resembles an isidium, but is formed as an outgrowth of the medulla rather than the cortex. It is associated with soralia of species in the genus Usnea.

### Schizida and thlasidia

The lichenologist Josef Poelt proposed new terms to distinguish certain isidia-like structures:
* Schizidia (introduced in 1965) are flattened pieces that form when the top layers of a lichen split; they are seen in seen in Baeomyces rufus.
* Thlasidia (defined in 1986) are cylindrical propagules that break off easily, similar to isidia; they are found in Gyalideopsis anastomosans.
In Heiomasia sipmanii, Aptroot et al. (2009) labelled disc-shaped isidioid structures as schizidia, differing from the definition given by Frisch and Kalb (2006) for Stegobolus in the family Graphidaceae. These structures in H. sipmanii and H. seaveyorum resembles thallus outgrowths more than true isidia, which have an upper cortex and photobiont layer. Terms like "pseudisidia" (isidioid structures without a cortex) and "pseudoisidia" (isidia-like formations devoid of photobiont cells) can cause confusion. The former refers to isidioid structures without a cortex, while the latter signifies isidia-like formations devoid of photobiont cells. Nelsen and colleagues suggest referring to these Heiomasia structures as isidia, as their lack of cortex is due to the thallus being ecorticate.
The debate over these terminologies persists, especially for ecorticate isidia-like outgrowths in lichens like Heiomasia seaveyorum. "Pseudisidia" and "pseudoisidia" are used interchangeably employed by different authors, adding to the confusion.
An isidioid soredium appears as a secondary corticate protuberance in soralia-like clusters. Polyisidia, clustered isidia formed on thalline outgrowths, are unique to the genus Pyxine. Thlasidia resemble pseudoisidia at their ends but contain photobiont cells in soredia-like patches at their bases. They originate from the thlasidium and are found only in the crustose, epiphytic lichen Gyalideopsis anastomosans.
Another unique structure is the Phyllophiale-type isidium, also called scutelliform isidia. This disc-shaped propagule, associated with the foliicolous lichen fungus Phyllophiale (now Porina), has a small Phycopeltis alga thallus surrounded by fungal hyphae. It grows via its hyphal fringe and algal filaments, forming a fungal network over adjoining algal thalli, eventually producing similar disc-like propagules elevated from the main thallus.

## Formation and development

Although the formation process varies among lichen species, in all cases isidia develop into small "miniature lichens" containing both fungal and algal partners, capable of independent growth. Often, isidia start when cells from the inner layers push up through the outer surface, bringing algal cells with them. Sometimes, the isidium sprouts directly from the outer cortex. Initially lacking a cortex, this forms later as the process matures. In other cases, isidia form from a protuberance of the thalline cortex, with tissue from the algal layer advancing into this protrusion. Sometimes, isidia originate when cortical hyphae ensnare free-living algae, which are then encapsulated by the outgrowing hyphae. Connections to the internal thallus layers are secondary in these cases. This process resembles cephalodia, highlighting the adaptability of lichen tissues.
Detailed anatomical studies of Parmotrema tinctorum and Parmelinopsis minarum have revealed a precise developmental sequence in isidium formation. In these species, the process begins with the proliferation of cortical cells, which creates a small protuberance on the thallus surface. This initial stage is followed by increased division of photobiont cells directly beneath the growing cortical layer. Only after this protuberance forms do medullary hyphae begin to intrude into the developing structure, growing and sometimes branching within it. In the final developmental phase, the base of the isidium constricts, ensuring the propagule remains attached until it is fully mature. This controlled development process helps ensure that isidia are only released when they are fully formed and capable of establishing new thalli. The size and structure of mature isidia contribute to multiple functions beyond reproduction – large isidia can significantly enhance a thallus's capacity for water absorption and retention, while their height allows them to remain above the water film on wetted thalli, facilitating continued CO$_2$ exchange. Similar developmental patterns have been observed in other Parmeliaceae species, suggesting this may be a common growth pattern in the family. Similarly, experimental studies on Pseudevernia furfuracea reveal that isidia formation involves high cellular turnover in both symbiotic partners, with increased asexual spore production in algae and proliferation of medullary hyphae. These isidia are metabolically active, showing heightened photosynthesis and dark respiration rates.
Some species show unique developmental patterns. In Peltigera praetextata, isidia form only at sites of cortical injury, which can be experimentally induced by making incisions on the thallus. Some researchers suggest calling these foliose outgrowths "lobuli" due to their regenerative role. In gelatinous lichens, isidia begins with the active division of algal cells at the thallus periphery. In non-corticate lichens, this triggers a small protuberance soon invaded by hyphae. In corticate lichens, increased algal proliferation stimulates cortical cell division, ensuring the thalline protuberance is uniformly coated with a cortical layer.

### Regeneration

Although many lichens do not rely exclusively on isidia for reproduction, these structures help "refresh" older thalli. On ageing thalli, isidia may sprout into new lobes. For example, a study on Parmotrema tinctorum found that isidia placed in nylon bags on Cryptomeria japonica trunks formed protuberances within six months, eventually maturing into lobules with rhizines and lichen compounds by the end of the year. Similarly, the isidia of Peltigera praetextata formed small juvenile thalli in 4–5 months when sown into soil, with larger thalli appearing after eight months.
It is commonly believed that after dissemination, isidia deconstruct into a loose association of fungal and algal cells before forming a new thallus. However, some species contradict this. Phyllophiale pastillifera isidia rapidly develop into a thallus shortly after detachment. Similarly, Lobaria pulmonaria isidia germinate directly into young thalli without a dedifferentiation phase.
While isidia are less conducive to dispersion than soralia due to their weight, their protective cortex makes them resilient in adverse conditions. Larger lichen fragments regenerate faster but are harder to disperse.
In the foliose lichen Parmelia saxatilis, mature isidia evolve into new thalline scales on the thallus surface. These emergent scales rejuvenate growth by overlapping older thalli like tiles, using the tissue from older, deceased lobes as a water reservoir.

### Transplantation studies

Lichen transplantation is a way to restore or introduce lichens where they have died out or never existed. Because isidia can serve as "starter kits" for new lichen growth, researchers sometimes collect isidia and attach them to a new surface, hoping they will take root. Isidia, given their role in lichen propagation, have become assets in such transplantation studies. For instance, isidia from the foliose lichen Xanthoparmelia tinctina were used in an attempt to rehabilitate an abandoned asbestos mine. The goal was to introduce lichens to cover and stabilize the surface, reducing the exposed asbestos surface area and thereby mitigating the dispersal of hazardous airborne fibres. Achieving robust colonization was a challenge, however, with running water and debris posing significant hurdles.
From a conservation perspective, Parmotrema crinitum isidia were combined with surgical gauze fibres to facilitate effective substrate attachment during transplantation. On the other hand, Sticta sylvatica required a considerable 24-month period post-transplantation to generate small lobules from its isidia.

## Taxonomic value

Erik Acharius first created the genus Isidium based on the presence of isidia. William Nylander later classified many Parmelia species by whether they possessed isidia. In 1924, Du Rietz ascribed taxonomic value to isidia and introduced the concept of species pairs, in which lichens appear morphologically and chemically identical but differ by the presence of vegetative diaspores. In some genera, isidia serve as important taxonomic markers. For example, Leptogium lichens can be differentiated by their isidium development and morphology, with two primary types: hirsutum-type and saturninum-type. Mature isidia in certain Leptogium species help distinguish superficially similar species due to their unique characteristics.
The value of isidial features in taxonomy is debated. Poelt noted that isidia formation in Collema, used by Gunnar Degelius to delineate varieties and forms, was "little stabilized morphologically or systematically". Kalb and Hafellner described Porina isidiata as the isidiate counterpart of specimens resembling P. atlantica, but regional observations suggest P. isidiata might be a variant of P. atlantica. In Porina, distinguishing genuine isidia from abnormal photobiont outgrowths is challenging. McCarthy (1993) and Harris (1995) suggested some structures result from aggressive photobiont growth. Certain species, like P. ocoteae, display isidioid outgrowths under stress. These observations indicate that "isidia" presence should not always be a defining taxonomic criterions, as their structure can be influenced by environmental factors.

## Ecology

Isidia significantly influence the ecological roles and physiological capacities of lichens. While attached to the thallus, isidia significantly increase surface area, enhancing gas exchange and overall photosynthetic capacity. Their dense structure, coupled with a polysaccharide-rich coating, helps them retain moisture and supports survival in fluctuating environments. However, this robustness also makes isidia heavier than lighter propagules like soredia, potentially limiting long-distance dispersal.
Beyond their primary reproductive role, isidia play a crucial part in complex symbiont dispersal networks. In addition to transporting the primary fungal and algal partners, they act as vectors for other microorganisms, including bacteria and secondary fungi. While the precise functions of these additional symbionts remain under investigation, their consistent presence suggests that isidia serve as comprehensive microbial packages rather than solely as reproductive propagules. Some lichens, notably Leptotrema and Graphina, rely on isidia for aeration; porous hyphal tissue at the isidial apex provides a transition to respiratory pores that become sealed by a secondary cortex after isidium detachment.

## Distribution

The morphological features and relative weight of isidia affect their dispersal range and consequently influence lichen distribution patterns. Their heavier structure limits long-distance dispersal compared to lighter propagules such as soredia, potentially contributing to challenges in habitat colonization, especially for threatened or endangered species in fragmented landscapes. Even suitable habitats located relatively close by can be difficult to colonize, possibly exacerbating population declines.
Historical field observations from South Africa's Cape Peninsula have documented multiple natural mechanisms for isidium dispersal. In seasonally dry watercourses, strong winds during dry periods carried isidia upwards, gradually colonizing higher-altitude areas. Additionally, cattle and antelope following watercourses, particularly in dry conditions when stream banks retained more succulent vegetation, likely transported isidia. Birds attracted to watercourses during the summer months similarly facilitated wider distribution by carrying isidia on their feet, much like seed dispersal in plants.
Vegetative propagules like isidia often inhabit a broader range of habitats than spores (e.g. ascospores or conidia), as they do not require resynthesis of the lichen symbiosis. Lichens producing these propagules frequently lack apothecia, or if present, apothecia are usually sparse or immature. About 25–30% of foliose or fruticose lichens produce isidia, highlighting their broad ecological role.

## Eponyms

In lichens, specific features related to their isidia are often captured in their species epithet. For instance, the epithet of Porina coralloidea alludes to its isidial features, while Aspicilia stalagmitica earned its name due to distinct isidia-like outgrowths.
Species epithets often directly incorporate the term "isidia" (or its variations), as seen in numerous examples like Niebla isidiaescens, Arthonia isidiata, and Porina isidiata. Astrothelium isidiatum, characterized in 2023, is notable as the only species of the large family Trypetheliaceae that is known to produce isidia.
Some species names combine "isidia" with descriptive terms to indicate specific characteristics:
* Compositrema isidiofarinosum: a powder-like (farinose), isidiate thallus.
* Rinodina densisidiata for its densely-covered surface
* Menegazzia caviisidia features hollow isidia
* Caloplaca squamuloisidiata for its squamulose isidia
* Pseudocyphellaria macroisidiata for its large isidia
* Menegazzia globoisidiata for its round (globose) isidia
* Acanthotrema alboisidiatum for its distinctive white isidia
* Fissurina longiisidiata for its long and cylindrical isidia
* Herpothallon viridi-isidiatum for its greenish-coloured pseudisidia
* Pertusaria xanthoisidiata for its yellow isidiata
* Ocellularia croceoisidiata for its yellow-gold isidia Porina monilisidiata for its moniliform isidia (i.e., resembling a string of beads)
* Porina ramiisidiata for its irregularly branched isidia.
Geographic distribution may also be referenced alongside isidial features, as in Neoprotoparmelia amerisidiata, N. australisidiata, N. brasilisidiata, and N. siamisidiata.

## History

Erik Acharius was the first to recognize the taxonomic value of coral-like outgrowths on lichen thalli. In 1794, Acharius defined the genus Isidium as "branchlets produced on the surface, or coralloid, simple and branched". It included densely isidioid species such as Isidium corallinum (now Lepra corallina) and I. westringii (now Pertusaria pseudocorallina). Genus Isidium has since been synonymized with Pertusaria.
Early researchers, such as the German botanist Friedrich Rosendahl, closely examined how isidia develop in different Parmelia lichens. For instance, In P. papulosa, which has a cortical layer one cell thick, isidia begin as small swellings or warts on the upper thallus surface. The cortex cells lose their normal arrangement and show irregular division, further dividing as photobiont and hyphae push up. The mature isidia in this species are cylindrical or clavate, simple or branched, and they minute rhizoids, a unique feature. The inner tissue matches the normal thallus with a distinct cortex, algal layer, and pith. Similarly, in Parmelia proboscidea, isidia develop into dark-coloured cilia.
In Parmelia scortea the cortex is several cells thick, with the outermost rows compressed and dead in older parts of the thallus. Isidia begin as minute warts, with lower layers of living cortical cells dividing actively. The protuberance formed pushes off the outer dead cortex and emerges as an isidium, which is stouter than those of P. papulosa and may be simple or branched. In this case, algae are scattered through the pith of the isidium rather than forming a definite zone.
The coralloid, branching isidia of Umbilicaria pustulata adorn the upper surface and margins of the thallus. They begin as small tufts of cylindrical bodies, sometimes broadening into leafy expansions with crisp edges. Typically situated on bulging pustules where intercalary growth is active, these isidia cause the tissue to become slack. The centre of the isidial tuft may fall out, leaving a hole that enlarges as the thallus grows. New isidia sprout from the wound edges, repeating the process. Friedrich Bitter demonstrated that these structures form due to isidial growth weakening the thallus, not from injury.
About 100 years ago, Du Rietz categorized isidia shapes as globose, cylindrical, claviform, or coralloid. The term isidium was first used in its current sense by Georg Friedrich Wilhelm Meyer [de] in 1825, and adopted by Elias Fries in 1831. In 1929, the British botanist Sidney Garside documented a distinctive type of isidium formation in Siphula tabularis. Unlike typical isidia that form as external outgrowths, these "endisidia" developed entirely within the photobiont layer, where groups of photobiont cells became surrounded by a cortex-like zone. These structures would later break through the cortex at the thallus tip and detach, leaving distinctive pits behind. This represented the first documented case of endogenous isidium formation in lichens.
The term isidium derives from the New Latin isidium, which comes from the Ancient Greek isis, Classical Greek: Ἶσις, meaning "coral", with the diminutive suffix idium. The adjectival form of isidia is isidiate, and isidiiferous refers to a lichen thallus bearing isidia.
A low-cost device was designed to trap microscopic from the air. Molecular techniques were developed to identify the captured propagules, capable of detecting a single asexual propagule, such as an isidium. This combined approach of using mechanized traps with DNA diagnostics represents a pioneering effort to study lichen dispersal.