The entire cap can be removed from the apex of the primary root of Zea mays. When this is done the nuclei of cells of the quiescent centre are stimulated to synthesize DNA and subsequently enter mitosis; the labelling and mitotic indices reach maximum values 12 and 18 h, respectively, after decapping. Division of cells of the quiescent centre and their descendants leads to the regeneration of a new cap which is completed about 3-4 days after decapping. At this time a new quiescent centre is also established. The development of a new cap is a consequence of the changes in the polarity of growth and division of cells at the apex. Quantitative data are presented of the changes in rates and planes of division of the cells involved in cap regeneration. The data suggest that cells of the quiescent centre, stimulated to divide by de-capping, complete about four mitotic cycles before a new quiescent centre forms. Experiments in which decapped roots were grown in a solution of colchicine, to induce polyploid nuclei, also showed that cells of the quiescent centre have the potential to enter four or five successive mitotic cycles as, in the quiescent centre of roots so treated, nuclei with up to 64C DNA content were found after 4 days.

Mature cells of the cap of intact roots contain nuclei with an 8-16C DNA content. In regenerating caps nuclei with an 8C DNA content were frequent on the fourth day after decapping. The pattern of [3H]glucose incorporation, as judged by quantitative autoradiography, by regenerating caps was compared with that by normal caps. On the fourth day of cap regeneration the patterns were similar. These two sets of results suggest that differentiation of the regenerating cap proceeds normally at the levels of both nucleus and cytoplasm.

It is suggested that the initials of the cap collumella constrain the growth of the cells that constitute the quiescent centre and this in some way slows their rate of mitosis. Removing the cap relieves the constraint to cell growth and allows cell division, thus providing the trigger for regeneration. The changes in the pattern of cells in the regenerating apex, that accompany the reorganization of a new cap and quiescent centre, are explained in terms of the stresses that the cells impose upon each other.