 In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word ("phloios") meaning "bark".The term was introduced by Carl Nägeli in 1858.Phloem tissue consists of conducting cells, generally called sieve elements, parenchyma cells, including both specialized companion cells or albuminous cells and unspecialized cells and supportive cells, such as fibres and sclereids.