The Root Apex of Malva Sylvestris. Ii. The Quiescent Center

Embryos and primary roots at various growth stages of Malva sylvestris were treated with H3‐thymidine to determine the presence of a quiescent center by autoradiography. The nearly complete absence of nuclear labeling in cells of the quiescent center at all stages of root growth suggested that it was established prior to germination. The quiescent center increased in size during growth, and the cellular changes in apical organization as identified in a related anatomical study occurred outside of its limits. These anatomical changes were caused by the production of rootcap cells by the initials of the secondary columella and outer cortex. Autoradiography confirmed that the secondary columella initials were not quiescent during all growth stages and the outer cortical initials were not quiescent in those growth stages in which the anatomical changes occurred. The concepts of the promeristem and protomeristem are discussed in relation to the quiescent center.