Crystal grow larger long certain directions, preferred on the extension of developed layers. In the figure on the right, position 1 is preferred over position 2, i.e., larger energy drop when atoms fill into position 1.
Growth rate is inversely proportional to the translation distance.
In the right figure, the vertical translation distance is less than the horizontal translation distance, thus, the growth rate in the vertical direction is greater than that in the horizontal direction. Thus, longer direction in crystal form reflect smallest unit cell axis. The direction of growth is perpendicular to accreting faces, i.e., new faces parallel to old faces. The slow growing faces will become the ultimate crystal faces. Will change to lineage when some of the empty space filled.