Arbuscules may arise as lateral projections from coils. The size and density varies enormously between host species. The cortex of these plants usually lack intercellular spaces. Click to enlarge 53k Formation of AM Colonisation of roots may arise from hyphae that emerge from quiescent or growing hyphae in soil or roots. The process of colonisation has two phases, primary and secondary.
Primary colonisation arises from inoculum unattached to a host root.
A hypha may grow more than 15 mm before reaching the root surface, and at a rate of up to 0.5 mm per day. Hyphae do not always grow directly to the root, though they often proliferate in the vicinity of the root surface. After contact, the hyphae may branch on the root surface of compatible plants, forming one or more appressoria. Usually a single appressorium subtends a penetration peg either between or through the epidermal cell, and the remaining appressoria senesce. Hyphae may ramify along the outer and inner epidermis without any further penetration.