Molds are tiny, multicelled organisms made up of branching filaments called hyphae (singular: hypha). Some of the hyphae are embedded in the material on which the molds grow and are called vegetative hyphae. Many molds also have other hyphae, called aerial hyphae, which absorb oxygen from the air. Rhizoids, hairlike filaments resembling roots, grow from the vegetative hyphae of some molds. Fragments of vegetative hyphae that break off can develop into new individual molds. Some molds develop filamentous runners, called stolons, which give rise to new individuals.
A mold normally reproduces by forming spores, either sexually or asexually. (A spore is a one-celled reproductive body.) Sexual spores are formed by the joining of two hyphae of the same or different molds. Asexual spores are produced by fruiting bodies, which, in the case of most molds, form at the tips of special types of aerial hyphae, called fertile hyphae.
The various species of molds differ in the types of asexual spores that they produce. Sporangiospores and conidia are the most commonly produced asexual spores. Sporangiospores are produced within a rounded structure called a sporangium. Conidia are long chains of naked spores, produced and supported by a swollen structure called a sterigma.
Most molds produce spores that are distributed by air currents. Water molds, which live in freshwater, produce spores that propel themselves through the water with whip-like structures called undulipodia. Depending on the species of mold, spores may be of various colors, including blue, green, and black. The edge of a mold growth is whitish while the center—the mature portion containing the spores—is pigmented.
Most molds belong to the divisions Zygomycota, Ascomycota, or Deuteromycota of the kingdom Fungi. Water molds belong to the phylum Oomycota of the kingdom Protista.