The fungus may form a loop or coil in this passage cell. In the absence of a hypodermis, the fungus penetrates between cortical cells. In Arum-type AM, the fungus elongates through the intercellular space, branching repeatedly in a pattern typical for each family. Initially, arbuscules are formed close to the penetration point. As the colony extends within the root, fresh arbuscules are formed in many of the cells.
Aged colonies tend to have fewer arbuscules.
If the fungus forms intraradical vesicles, these will be appear soon after initiation of the unit. They tend to be formed in specific locations in the root and at specific times of the life cycle indicating that the processes are under host and fungal genetic control. Soon after initiation of the colonisation unit, a lateral emerges from hyphae near the appressorium. The lateral elongates along the root and establishes a second colonisation unit. Secondary colonisation commences 10 to14 days after initiation of primary colonies.