Final answer:
The function of the raphe in diatoms is to allow them to attach to surfaces or propel themselves in one direction.
Step-by-step explanation:
The function of the raphe in diatoms is to allow them to attach to surfaces or propel themselves in one direction. The raphe is a slit in the silica shell of diatoms, and by expelling a stream of mucopolysaccharides from the raphe, diatoms can create a sticky substance that helps them adhere to surfaces. This ability to attach to surfaces is especially important for diatoms in environments with strong currents.