20% (one in five) of fungal species live in lichens. Lichen fungi belong to several different groups of fungi. Most common (over 40%) are Ascomycetes, which produce spores in a sac-shaped container, the "ascus". Only a few types of alga occur within lichens; these algae also have their own names. Algae may be sometimes the dominant partner in so-called 'jelly lichens', "Collema" and "Leptogium", but this is rare. More usually, the algae are the green alga "Trebouxia". Other species are the orange "Trentepohli", and the cyanobacterium "Nostoc".