Based on morphological criteria, fungi are often separated into two groups: yeasts and filamentous fungi. Uncellular fungi are known as yeasts. The extraction of nucleic acids from yeasts cells is described in Chapter 9. Filamentous fungi form a mycelia consisting of multinucleate, tubular hyphae, which may be separated into compartments by septa. Some fungi are dimorphic, that is, they can exist in a unicellular or multinucleate form. The switch from filamentous to pathogenic yeast form is temperature dependent in the human dimorphic pathogens 'Coccidioides immitis', 'Blastomyces dermatitidis', 'Histoplasma capsulatum', and 'Paracoccidioides brasiliensis'. In the dimorphic plant pathogen, 'Ustilago maydis', the filamentous form, which is produced by mating, is infectious while the fungus grows in culture as a yeast. Some groups of fungi belonging to the phylum Chytridiomycota do not form a true mycelium. Unlike other fungi, chytrids produce motile spores possessing a flagellum.