Fungi are generally saprobes or parasites, except lichens, which rely on symbiotic algae. They have cell walls and feed by external digestion and absorption. Most develop into massed filaments called hyphae, which grow into dead or living material, but some have reverted to single cells such as yeasts.
Even larger fungi like mushrooms are generally inconspicuous except for spore-forming stages. In a few kinds the spores have a posterior flagellum, but most are non-motile and rely on wind, water, or animals for distribution. In sexual reproduction the nuclei usually do not fuse until just before spores are formed.