title: Collema tenax
text: # Collema tenax
  
  Collema tenax is a species of lichen known by the common names jelly lichen, black lichen, and desert lichen. It occurs in Arctic and temperate regions in the Northern Hemisphere. It is very common in North America, and it is present in Europe, Asia, and Africa. It is the most widely distributed Collema.
  
  This lichen has a thick black or dark green gelatinous thallus which is variable in size. It is generally 1 to 3 centimeters wide. It contains a symbiont, the cyanobacterium Nostoc commune. It reproduces when pieces of the thallus break off, or when isidia, soredia, and apothecia are produced. It is tolerant of desiccation, shrinking when dry and swelling up when water is present.
  
  This lichen is terricolous, growing on the soil surface. It is an indicator of basic soils, and is often found on gypsum-rich and other calcareous soils. It is a common component of cryptobiotic soil crusts, for example, in the dry regions of the western United States. It fixes nitrogen, increasing the nitrogen levels in the soil.
  
  