Lord Rayleigh has shown that there is a tangential motion as well as a motion in and out.
Within the crust of the earth, whether by the contraction of the interior or in any other way, tangential pressures were set up. Since the crust is not of uniform strength throughout, only the weaker portions yielded to the pressure; and these were crumpled up against the more resisting portions and sometimes were pushed over them.
The parenchyma is often arranged in tangential bands between the layers of sievetubes and tracheal elements.
This is called the tangential area, and will be denoted by T1.
Secondary growth in thickness is effected by the tangential division of superficial cells.