Cell Production by Root Caps

SUMMARY Four independent methods show that the cell‐doubling time of the first tier of cap initials is about 10 h in roots of Zea growing at 23°C. By the fourth tier the cell‐doubling time lengthens to about 90 h due to cells going out of cycle though these can go on synthesizing DNA. The first four tiers produce enough cells to renew the cap in from 22 to 33 h. This high rate of cell production is confirmed by observing the transit of cells marked by caffeine along the length of the cap. Although some cells reach the tip of the cap quickly, others remain within the meristem for 24 h without dividing. The results are discussed in relation to work on other plants and to the anomalous cell cycles of the cap meristem of Zea.