Abstract
Crown root primordia in stem of rice plants (fig. 9) originate from certain cells derived from the innermost ground meristem cells adjacent to the continuous ring of meristematic tissue which will differentiate to the peripheral cylinder of vascular bundles and fibers (PV in fig. 9). The vascular tissue between the crown root and the peripheral cylinder of the longitudinal vascular bundles of the stem originates also from the same cells. Before the crown root initiation, several cell layers are formed by one or two periclinal divisions of the innermost ground meristem cells (fig. 4). In these several cell layers, subsequently, the cells of the inner one or two layers bigin to divide transversely while the cells of the outer one or two layers bigin to divide radially somewhat later (fig. 5). At this stage, some groups of cells from the inner cell layers elongate radially and the nuclei and the nucleoli of the cells become enlarged (fig. 6). These groups of cells are the initial cells of crown root primordia as observed under ordinary microscope, also the density of the protoplasm of these cells becomes gradually higher. The same phenomena were observed in the outer cells adjacent to the crown root initial cells. These outer cells also constitute the primordia. In this stage, the initial cells bigin to divide periclinally, transversely and radially to form the crown root primordia (fig. 7). On the other hand, the inner cells which have not differentiated to crown root primordia undergo transversal division and sometimes periclinal division (fig. 8). They give rise to the vascular tissue between the crown root and the vascular system of the stem. When the sequence of the region where the initiation of crown root primordia is taking place is observed basipetally in longitudinal sections, it was found to correspond to the same region where the future sclerenchymatous elements in the peripheral cylinder of vascular bundles and fibers bigin to elongate (fig. 6). The differentiation of the peripheral cylinder of vascular bundles and fibers proceeds acropetally, these regions may migrate acropetally as the stem develops. When the distribution of the crown root primordia formed in the main stem is examined, two "primordium forming zones of crown roots" (fig. 10) were recognized in the region between the axils of Pn+1 and Pn leaves (fig. 9). These zones were observed to have a relationship to the vascular pattern of the stem. In the region between these two "primordium forming zones of crown roots" where there are few crown root primordium formations, the meristematic ring of the future peripheral vascular bundles and fibers is separated by both larger and smaller leaf traces observed in transversal sections (fig. 11).
