Mycorrhizal fungi are well known for increasing nutrient uptake but their effects on soil physical structure and water flow are less well understood. Here I explore what we know about the physical structure of mycorrhizal external mycelia and examine how that physical structure affects plant water uptake and reverse hydraulic lift in unsaturated soils. Mycorrhizal fungi are structured such that there are linear cytoplasmic units that can extend for a meter or more. Cell membranes may be only located in hyphal tips within the plant and externally several centimeters to meters distant from the plant root. Individual hyphae form a linear surface that goes across soil pores increasing the tortuosity factor (Γ) of the pathway for water flow, thereby increasing conductivity. But hyphae are small in diameter, providing only a small surface area for that transport. Little about the reverse flows (hydraulic redistribution from plant to fungus) is known other than that they occur and could play a critical role in sustaining hyphae through drought. The ultimate importance of mycorrhizae in plant–water relations depends on the drying patterns, the soil pore structure, and the number of hyphal connections extending from the root into the soil. New technologies are needed to adequately parameterize models of water horizontal flow patterns to: (i) observe and monitor the growth of roots and mycorrhizal fungi in situ; and (ii) describe the localized environment at high temporal and spatial resolution.

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