Lichen

One of a class of cellular flowerless plants technically called Lichenes having no distinction of leaf and stem usually of scaly expanded frond like forms but sometimes erect or pendulous and variously branched They derive their nourishment from the air and generate by means of spores The species are very widely distributed and form irregular spots or patches usually of a greenish or yellowish color upon rocks trees and various bodies to which they adhere with great tenacity They are often improperly called rock moss or tree moss