The study of fungi must be regarded as a side trip in the study of evolution on earth, because the ancestors of fungi are uncertain, and the evolutionary future is no clearer. Yet they cannot be ignored. They are both a boon and bane to our species. While co-evolving with the rest of life on earth, some fungi became symbionts of considerable benefit. Others became parasites, causing disease to plants and animals. Still other fungi joined the prokaryotes in the niche of decomposer, a role as important to us as any other. The fungi are primarily made up of filaments that lie fairly loosely as in molds, but may weave intricate structures like mushrooms, morels and cups. Their filaments, called hyphae, vary in the degree of septation or may lack septation. Two of the phyla, Ascomycota and Basidiomycota, have unique modes of sexual reproduction involving a coexistence of nuclei from different organisms within a single cell without fusion. In ascomycetes, the fruiting structure may be woven from filaments of different nuclear content. Some fungi form symbiotic associations with photosynthetic protists, cyanobacteria or plants. Two of the groups of fungi are aggregations of fungi of diverse kinds and dubious affinity. The Fungi Imperfecti are so designated because their common trait is the absence of a sexual phase. The yeasts, too, have various affinities. Their common characteristic is unicellular habit. The uniqueness of fungi leaves little doubt as to why they are a separate kingdom.