Arbuscular fungi are obligate biotrophs that are unable to sustain growth and reproduction apart from a plant host. This restrictive niche appears to be balanced by an extremely wide host range, so that ample opportunities exist for any dispersed fungal propagule to establish a new individual somewhere else. The filamentous habit characteristic of this and other fungi affords organisms considerable versatility in their life cycles, with the capability of indefinite growth as long as carbon from a host plant source is available. All indications are that reproduction is clonal and that spores formed asyncronously as mycorrhizal colonization progresses are somatic offshoots compartmentalizing a varying number of nuclei. Below are the various discrete structures produced by a mycorrhizal fungus as it becomes established, grows, and reproduces in a host root.

Intraradical hyphae originated from a single entry point appear to have limited growth, forming an "infection (or colonization) unit" of a size regulated by host-fungus interactions. The external hyphae are of various morphologies and functions, ranging from "infective" hyphae to "absorptive" hyphae to "fertile" (spore-bearing) hyphae. Duration of growth, senescence, etc. and other properties of each hyphal form is poorly understood at this time. It is the infective hyphae which initiate new points of colonization on the same root, other roots of the same plant, or roots of adjacent plants. Work with pot cultures indicates external hyphae (fragments or attached to mycorrhizal roots) are most infective in the families Glomaceae and Acaulosporaceae and least infective in the family Gigasporaceae.

These "tree-like" structures originate from branches of the intraradical hyphae after the branch hypha penetrates through the cortical cell wall. An arbuscule forms between the cell wall and plasma membrane. Although of short duration, arbuscules can appear numerous in roots over a long period of time as long as the fungus is growing and producing new ones. Arbuscular differentiation appears to persist much longer (especially in pot cultures) in roots of plants colonized by species in the suborder Gigasporineae (3-4 months) than in the suborder Glomineae (2-3 months). Arbuscular morphology also differs among fungi in the three families (for example, compare those of Scutellospora heterogama with those of Glomus diaphanum or with those of Acaulospora denticulata).

Thin-walled lipid-containing bodies produced terminally or intercalarily from hyphae in the root cortex by species of fungi in the suborder Glomineae. In the family Glomaceae, vesicles generally are ovoid to ellipsoid (e.g., G. diaphanum), whereas those in the family Acaulosporaceae often are ellipsoid to irregular and/or knobby (e.g., E. colombiana). Vesicles differentiate very early in mycorrhizal development in some members of Glomus, but generally proliferate at approximately the same time sporulation is triggered and increase thereafter. Late mycorrhizae (70-90 days in pot cultures) show few arbuscules, but abundant intraradical hyphae and vesicles. Glomus mosseae and related species (e.g. G. caledonium) produce small hyaline thin-walled structures resembling vesicles on germ tubes (in soil and agar cultures) and external hyphae. They may be analagous to auxiliary cells in Gigasporaceae (see below).

Clusters of thin-walled cells which branch from extraradical hyphae of fungi in the suborder Gigasporineae. Cell surfaces have a spiny surface in the genus Gigaspora (e.g., Gi. albida), which are reduced to knobs or an almost smooth surface in various groups in the genus Scutellospora (e.g., S. pellucida). Auxiliary cells often differentiate on germ tubes from spores prior to establishment of any mycorrhizal colonization (including spores germinated on agar media). They appear to peak at or within a short period after onset of sporulation in pot cultures. They often are in low numbers or absent in pot cultures over 4 months old in age.

Spores are differentiated either in soil or in roots in all genera except Gigaspora. Even though all members of Glomales are classified in the Zygomycota, none are known to produce zygospores and their asexual spores have few (if any) morphological affinities with asexual sporangiospores or chlamydospores. Intraradical sporulation, when it occurs, is most abundant in some Glomus species, the most notorious (and consistent) species being G. intraradices and G. diaphanum. It is a myth that arbuscular fungi behave similar to many other fungi by initiating sporulation after nutrient deprivation (host stress in obligate symbionts). Timing of onset of sporulation varies with species and also growing conditions (affecting fungus directly and indirectly through host physiology). It often occurs within 3-4 weeks after onset of mycorrhizal colonization under almost any conditions except high phosphorus in soil (which inhibits all phases). We, and researchers in Lyn Abbott's lab, have experimental results indicating sporulation occurs only after a critical threshold level of fungal biomass is established in roots. Sporulation then progresses asynchronously thereafter along with continued mycorrhizal development. In pot cultures, sporulation ceases with root growth in species of Glomaceae and Acaulosporaceae but may continue to occur at low levels in species of Gigasporaceae. Spores of all arbuscular fungi are assumed to be infective once they form one or more germ tubes. However, those of fungi in Gigaspora are the most infective of all genera if spores are in a healthy condition (we can obtain 100% single-spore cultures from Gi. gigantea, for example).