thumb|Illustration showing cephalodium in relation to other parts of the lichen:1 – Cephalodium 2 – Cyanobacteria 3 – Cortex 4 – Green algal photobiont 5 – Medulla A cephalodium () is a small gall-like structure found in some lichens. They occur only in lichens which contain both cyanobacterial and green algal partners. Cephalodia can occur within the tissues of the lichen, or on its upper or lower surface. Lichens with cephalodia can fix nitrogen, and may be an important contributor of nitrogen to the ecosystem.
thumb|Illustration showing cephalodium in relation to other parts of the lichen:1 – Cephalodium 2 – Cyanobacteria 3 – Cortex 4 – Green algal photobiont 5 – Medulla A cephalodium () is a small gall-like structure found in some lichens. They occur only in lichens which contain both cyanobacterial and green algal partners. Cephalodia can occur within the tissues of the lichen, or on its upper or lower surface. Lichens with cephalodia can fix nitrogen, and may be an important contributor of nitrogen to the ecosystem.
== Context == Lichens are complex organisms composed of a fungal partner and a photosynthetic partner. While the photosynthetic partner is most often a species of green alga, in about 10 percent of all lichens, a species of cyanobacterium is involved instead. In an even smaller number of cases – estimated at between 2 and 4 percent of all lichens – species of both a green alga and a cyanobacterium serve as photosynthetic partners. There are roughly 520 species of these "tripartite" lichens, which fall into at least 21 different genera. In most of these lichens, the green algae live within the lichen's medulla while the cyanobacteria are housed in specialized structures called cephalodia. These can be located on the upper or lower surface of the lichen, or within its interior. In a few species, the cyanobacteria is the main photobiont and the green alga is housed in the cephalodia.
Discovered by embedding cosine similarity (sentence-transformers MiniLM, 384-dim).