Diatom morphology

Diatom cells are described in two views – valve and girdle view.

Cells of A. pellucida are narrowly rectangular in girdle view, and needle-shaped in valve view.

They are 80--140µm long (0.08--0.14mm), 7--9µm wide in valve view, and about 6µm wide in girdle view

Girdle width varies with the stage of the cell cycle.

The raphe slits are relatively short, 15--20µm long, near the ends of the valves. Each slit is flanked by longitudinal ribs that connect to a narrow rib that extends through the centre of the valve.

There are parallel rows of pores crossing the valve surface – 37--40 rows in 10µm - and the pores in the rows are about 40 in 10µm.

The girdle region is formed of several open bands that also contain pores.

Diatom ultrastructure

More detail of the diatom wall structure can be seen when an electron microscope is used.

External views show the short raphe slits, situated towards the valve apices. The regular pores have slit-like external openings.

Internal views of the valves show the ribs surrounding the short raphe slits, with a long narrow rib through the centre of the valve.

The pores are more rounded and have an occlusion which is even more finely perforated - the sub-pores are so small they can only be resolved with transmission electron microscopy.


Raphe slits are longitudinal slits in the silica wall along the axis of the valve, allowing contact between the surface of the protoplast and the external environment. Diatoms require raphe slits to be able to move actively over surfaces.