Abstract
The formations of the crown root primordia in the unelongated stem portion of rice plant at 21 days after heading were investigated by light microscope as every serial cross sections with consideration to the position of crown root primordia. Our detailed anatomical observations revealed that regardless of the rooting bands, the root primordia were most frequently formed just below the nodal plate and rarely formed above the nodal plate, with changing numbers of root primordia successively throughout all the unelongated stem portion above around the 4th node. In general, the root primordia were formed more in number, externally in the upper portions of the stem, and internally in the portion where peripheral cylinder of longitudinal vascular bundles (PV) was not divided by the large vascular bundle of leaf sheath and various vascular bundles such as nodal vascular anastomoses developed. These results did not fit the well-known 'Nodal root' and the 'Shoot unit' related to the position of root primordia initiation, hence, we propose the new 'unit' consisting of three positions, that is, 'Divided Portion (DP)I', 'DP II', and 'non-DP' by considering the form of the PV in cross sections.
