FIGURE 21-20 Structures of the Arabidopsis thaliana shoot and root meristems. (a) Transverse section through the apex of the shoot apical meristem. The organizing center cells signal to maintain the overlying stem cells. The stem cells produce daughters by division in the direction of the black arrows, generating rapidly dividing transient amplifying cells that will eventually differentiate and give rise to entire organs, such as a leaf. (b) Transverse section through the root meristem. Stem cells surround the mitotically less active quiescent center, four cells that send signals to prevent stem-cell differentiation. Each stem cell divides asymmetrically: one daughter remains adjacent to the quiescent center and becomes a stem cell (self-renewal); the other daughter becomes a transient amplifying cell that divides a number of times before exiting the cell cycle, elongating, and assuming a specific differentiated state. See R. Heidstra and S. Sabatini, 2014, Nat. Rev. Mol. Cell. Biol. 15:301.