A hyaline thickening of silica at the valve apices in some pennate species. The genus Sellaphora possesses variable development of polar bars.
Some authors use "transapical thickening", rather than polar bar.
Image Credit: Marina Potapova
Scanning electron micrograph of Sellaphora wallacei showing the polar bar at the apex.
Light micrograph of Sellaphora rexii, with the thickening of the polar bar appearing lighter than other parts of the valve.