Septum

A septum (plural: septa) is a cross-wall that divides a hypha into individual cells. In septate fungi — which include most Ascomycota and Basidiomycota — septa occur at regular intervals along the hypha, creating a chain of cells. Each septum typically has a central pore that allows cytoplasm, organelles, and even nuclei to flow between cells, maintaining the hypha as a functionally connected unit despite the physical partitions.

Not all fungi have septa. Coenocytic fungi — including many Zygomycota and Chytridiomycota — grow as continuous tubes with no cross-walls, their hyphae containing many nuclei in a shared cytoplasm. The distinction between septate and coenocytic growth is one of the basic structural differences among fungal groups (see fungal taxonomy). Septate hyphae can seal their pores in response to damage, isolating injured cells and preventing cytoplasmic loss from spreading through the network. This gives septate fungi a damage-control mechanism that coenocytic fungi lack.

The septal pore is more than a simple hole. In Basidiomycota, the septum forms an elaborate structure called a dolipore septum, flanked by membranous caps (parenthesomes) that regulate traffic through the pore. In Ascomycota, the pore may be associated with Woronin bodies — dense protein structures that plug the pore when a cell is damaged. These structures reflect the tension between two requirements: connectivity (cytoplasmic streaming that distributes nutrients and signals across the mycelial network) and compartmentalization (the ability to isolate damaged or specialized cells). The septum negotiates this tension — a structural compromise between the network’s need for integration and its need for local autonomy.

Related terms

• Hyphae — the filaments that septa divide into cells
• Mycelium — the network in which septate hyphae function
• Anastomosis — hyphal fusion, which creates cross-connections between septate filaments
• Chitin — the cell wall polymer from which septa are constructed
• Fungal Taxonomy — septate vs. coenocytic growth distinguishes major fungal groups