ABSTRACT
This chapter discusses the distribution, structure, and function of the external hyphae of vesicular-arbuscular mycorrhizal (VAM) fungi. The most widespread fungi in the rhizospheres of Angiosperms are Zygomycetes that form symbiotic associations with plant roots -the VAM fungi. Root colonization by VAM fungi has both an internal and an external phase. The VAM fungi vary considerably in their innate capacity to produce external hyphae. Improved uptake of poorly mobile ions from the soil is the major benefit of VAM fungi to plants. A tenable model for enhanced soil stabilization by VAM fungi is that the external hyphae, along with plant roots, produce a framework for aggregation, while bacterial polysaccarides cement the soil particles together. Spores, infected root pieces, and hyphae may serve as sources of inocula of VAM fungi. Most researchers who attempt to quantify external hyphae of VAM fungi use methods that do not differentiate between active and inactive or dead hyphae.
