DEVELOPMENTAL STUDIES ON EUPHORBIA ESULA L.: APICES OF LONG AND SHORT ROOTS

Abstract

In both indeterminate long roots and determinate short roots of Euphorbia esula L., the promeristem usually consists of three meristematic layers. One of these represents the stelar pole, the second is continuous with the cortex, and the most distal appears to initiate the root cap – epidermis complex. Variations in this pattern are noted. Autoradiographs of apices of roots supplied with thymidine-H³ under field conditions demonstrate the existence of a region of low nuclear incorporation during a 24 or 48 hour period at the summit of the root axis in long roots. This region corresponds to the quiescent center described by previous authors. Short root apices consistently lack such a region. The pattern of nuclear incorporation of thymidine-H³ suggests that cell division is infrequent or absent in the center of the apex of long roots but not of short roots. Quiescence is thus a phenomenon which is superimposed upon the organization of the root apex in this species, and its presence or absence seems to be related to the developmental potentiality of the root. In many long roots the size of this quiescent region exceeds that of the region which may reasonably be designated the promeristem.