Primordia A & C are convergent, that means, contact parastichy appears.
Primordia D & B are divergent, that means, contact parastichy disappears.

Arrow show the moment of touch of two (younger A' and older C') primordia, moving in the opposite corners of the primitive unit cell A'B'C'D'. At that moment new vector A'C' arises.

Vector A'C' = Vector A'B' + Vector B'C'