Apical axis

The long axis of the valve face of a pennate diatom, passing through the apices. Follows the midline of the valve and may be curved, as in the genus Cymbella. See Pervalvar axis and Transapical axis.

Apical Axis
Image Credit: Marina Potapova
Navicula is symmetrical to the apical axis. The black line runs along the apical axis.