CELL PRODUCTION BY ROOT CAPS

Four independent methods show that the cell-doubling time of the 1st tier of cap initials is about 10 h in roots of Zea [mays] growing at 23.degree. C. By the 4th tier the cell-doubling time lengthens to about 90 h due to cells going out of cycle although these can go on synthesizing DNA. The 1st 4 tiers produce enough cells to renew the cap in from 23-33 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.