An isidium is a vegetative

Vegetative

Vegetative describes vegetation.Vegetative may also refer to:*Vegetative reproduction, a type of asexual reproduction for plants*Persistent vegetative state, a condition of people with severe brain damage...

 reproductive structure present in some lichen

Lichen

Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...

s. Isidia are outgrowths of the thallus surface, and are corticated (i.e., containing the outermost layer of the thallus), usually with a columnar

Columnar

In biology, columnar refers to the shape of epithelial cells that are taller than they are wide. Form follows function in biology, and columnar morphorphology hints at the functions of the cell. Columnar cells are important in absorption and movement of mucus...

 structure, and consisting of both fungal hypha

Hypha

A hypha is a long, branching filamentous structure of a fungus, and also of unrelated Actinobacteria. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium; yeasts are unicellular fungi that do not grow as hyphae.-Structure:A hypha consists of one or...

e (the mycobiont) and algal

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

 cells (the photobiont). They are fragile structures and may break off and be distributed by wind, animals, and splashing raindrops. In terms of structure, isidia may be described as warty, cylindrical, clavate (club-shaped), scale-like, coralloid (coral-shaped), simple, or branched.

Examples of isidiate lichens include members of the genera Parmotrema

Parmotrema

Parmotrema is a genus of lichen belonging to the family Parmeliaceae.-External links:*...

and Peltigera

Peltigera

Peltigera is a genus of approximately 91 species of lichenized fungi in the family Peltigeraceae.Commonly known as the dog lichen, lichens of Peltigera are often terricolous , but can also occur on moss, trees, rocks, and many other substrates in many parts of the world...

.