Isidium
An isidium is a tiny, wart- or finger-like outgrowth on the thallus surface of certain lichen species. It is one of two principal types of vegetative reproductive structures in lichens, the other being soredia. Each isidium contains both fungal and algal partners and is wrapped in a thin protective layer, distinguishing it from soredia, which lack this covering. While both function in vegetative reproduction, the heavier, corticate structure of isidia means they tend to establish in microhabitats close to the parent thallus, often favouring stable, humid niches where mechanical protection improves survival. Unlike spores, which are microscopic and easily carried over long distances by wind, isidia are larger, multicellular fragments that rely on external forces such as wind, rain, or animal contact, but typically disperse over much shorter ranges. 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,, or hollow.
Morphological characteristics of isidia are taxonomically informative, and often serve as distinguishing traits for species identification. Certain specialized forms, such as and, reflect subtle developmental distinctions. Ecologically, isidia increase the thallus surface area, which enhances moisture retention, gas exchange, and photosynthetic capacity. Functionally, isidia act as vegetative propagules—units of asexual reproduction containing both symbiotic partners.
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, cylindrical, scale-like, coral-like, club-shaped, disc-shaped, cup-shaped, and wart-like. Although they are always small, their surface may be smooth or knobby, shiny or matt. Most isidia are solid, but some lichens feature hollow, inflated isidia. In some lichens, soralia—defined areas on the thallus where soredia are produced—and isidia can form in overlapping locations. Soralia may grow at the tips of isidia, and isidia can sometimes develop within a soralium, where the loose network becomes compact and forms a secondary cortex.Some lichen species produce especially large, morphologically distinctive, or diagnostically important isidia that aid in species recognition. The isidia of the foliose lichen Pseudocyphellaria horridula are distinct, emerging vertically from the 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, can be difficult. Older isidia sometimes resemble tiny lobes 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, increasing the surface area.
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 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 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.Isidia can arise in different anatomical contexts. In some species, they originate internally beneath the and later emerge, developing a cortical layer as they mature—these are structurally. In others, they form as projections directly from the existing cortex and retain continuity with it; these are, with distinct internal layering.
A few species displayed isidiiform areolae, which are more or less spherical to cylindrical, and easily detached or broken. Examples include Caloplaca herbidella and Placynthiella icmalea. The latter's isidia-like areolae were termed "", as they resembled but remained attached at the base to the substrate. The entire areola of Placynthiella icmalea was identified as a blastidium.
An 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:- are flattened pieces that form when the top layers of a lichen split; they are seen in Baeomyces rufus.
- are cylindrical propagules that break off easily, similar to isidia; they are found in Gyalideopsis anastomosans.
The debate over these terminologies persists, especially for ecorticate isidia-like outgrowths in lichens like Heiomasia seaveyorum. "Pseudisidia" and "pseudoisidia" are used interchangeably 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 isidia. This disc-shaped propagule, associated with the foliicolous lichen fungus Phyllophiale, 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 boost a thallus's capacity for water absorption and retention, and their height allows them to remain above the water film on wetted thalli, facilitating continued CO2 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. After detachment, the fate of isidia varies among species. 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.
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 begin 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.