What cross-walls divide the hyphae of many fungi into cells? The answer is septa. A septum is an internal wall that forms across a hypha, dividing its long tube into cellular compartments; septa is the plural form.
Septate hyphae are not sealed into completely independent units. Septa commonly contain pores large enough for cytoplasm, organelles, and sometimes nuclei to pass between neighbouring compartments. In Basidiomycota, specialized dolipore septa help regulate this internal movement.
The compartments allow damage to be limited. If a hyphal region is injured, nearby septal structures can help restrict the loss of cytoplasm, while the remaining mycelium continues growing. New septa are generally formed behind the advancing tip as the hypha elongates and branches.
Not all fungi have septa. Coenocytic or aseptate hyphae lack regular cross-walls and contain many nuclei in a continuous cytoplasmic compartment. The distractors describe structures from other contexts: cilia are motile projections, villi are absorptive projections in animals, and pili are surface appendages of bacteria.