Perturbation of meristems of adventitious roots of Pistia resulted in abnormal patterns of cell division in the quiescent centre including the pole of the normally discrete epidermis. Two of the methods used, radiation from incorporated tritiated thymidine and cold treatment, also produced abnormal patterns in the lateral primordial present within the apices of the adventitious roots. Cells of the monostromatic epidermal shell divided periclinically in some roots and some primordia where all previous divisions would have been anticlinal. In other meristems, the normally quiescent cells of the stelar and cortical poles grew and divided to displace the former epidermal pole and cap initials.
Recovery from the perturbation resulted in the organization of a meristem which assembled a new monostromatic epidermal layer apparently discrete from the new cap initials below and the new cortical pole cells above. The new epidermal pole formed continuously with the old and, though the pattern of planes and rates of mitosis initiating and maintaining it must be determined as in normal ontogeny, some additional influence of the epidermis left intact above the region of abnormal activity appears to operate.