Abstract
Along the successive "shoot units" (abbreviated as SUs) of main axes of rice plants, the radial and the axial growths of stem parts were examined in relation to the diameters and the number of primary roots formed on them. The root forming zone (designated as the circumference of stem vascular cylinder, on which primary roots initiate) of each SU reached its final diameter successively from the 5th SU upwards to the 10th, in accordance with the plant development. In each SU, the later thickening period of the root forming zone coincided with that of the root primordia from the initiation to the attainment of their final diameters. And the diameters of matured primary roots were closely correlated with the concomitant increase in the diameter of the root forming zone of the same SU. From the 11th SU to the 13th, however, root forming zones reached their final diameters at the same time which coincided with the beginning time of the remarkable internode elongation. Consequently, their final diameters tended to decrease acropetally along the successive SUs. The similar tendency to decrease towards the 13th SU was observed in the diameters of primary roots. From these results, it is inferred that a causal correlation might exist between the diameter of root forming zone and those of primary roots formed on it. Throughout the SUs mentioned above, there was a close correlation between the number of primary roots and the size of the root forming zone at the time of primary root initiation.
