Lichens are usually presented as a classic example of mutualism , where each partner benefits from the relationship, but some authorities believe that this relationship might be a form of controlled parasitism. Certain fungi are also symbiotic with the roots or gametophyte stages of many types of plants. The body of a fungi consists of many small threads called hyphae , which intertwine to form a larger body called a mycelium . The cell walls of fungal hyphae are made of polysaccharides, like those of plants or bacteria, but they also contain chitin . Unlike strands of algae, the cross walls or septae between the cells in fungal hyphae are often incomplete, or lacking altogether.
Fungal strands are therefore often multinucleate .
Fungal hyphae only form a complete cross wall at the very tip of a hyphae undergoing sexual reproduction. This lack of cross walls may be the secret behind the evolutionary success of this widespread group of organisms. Fungi can move materials like nutrients and proteins back and forth very quickly by cytoplasmic streaming . Fungal digestion is extracellular, with the hyphae secreting powerful enzymes to digest the host tissue, then absorbing the breakdown products through their cell walls. Organisms that feed in such a fashion are called saprobes.