Lichen

Some people pronounce it 'litchen' to rhyme with kitchen but more people pronounce it 'liken'. 

There are  20,000 different species of lichens worldwide:  some are crusty (crustose), some are leafy (foliose) and some are branching like miniature fruit trees (fruticose).  The more structurally complex lichens require cleaner air, so they can be used as an indicator of air quality.  If you only find crusty lichens you are in an area which is relatively more polluted.  Leafy lichens indicate slightly better air quality and fruticose lichens indicate even cleaner air.

My photos show leafy lichens on a piece of ash wood, recently fallen from a tree at Filnore Woods.  Lichens have no roots so they do no damage to trees; they just find tree bark a good place to sit.

They are strange and fascinating organisms.  They look a bit like mosses but they are not.  A lichen is made up of a fungus and an alga living together and dependent on each other. (Sometimes a 'cyanobacterium' or 'blue-green alga' joins in too.) The fungi provide the structure and the algae use sunlight and carbon dioxide to provide food for the team by photosynthesis.  They are not just side by side, their cells are intermingled.  It's a totally symbiotic relationship.