MORPHOLOGICAL-STUDIES ON THE REGULARITY O F SHOOT DEVELOPMENT IN RICEPLANTS .7. THE MECHANISM TO CONTROL THE START OF INTERNODAL ELONGATION AND THE REGULAR DISTRIBUTION OF ELONGATED INTERNODES ON STEMS

The distribution of elongated internodes on stems in each plant was co mpared among the main shoots and tillers, and GA(3) was applied to the plant to determine the primordial internodes which are sensitive to g rowth substances. The results were as follows:1)The main shoots had fi ve or six elongated internodes. The total leaf number was one less on shoots with five internodes than on shoots with six internodes. 2)The position of the lowest elongated internode was the same on each tiller and the main shoot, or it was one internode lower on tillers than on the main shoot. 3)The number of elongated internodes on each tiller wa s five or six. 4)The time lag between the first bract (b(1)) initiatio n on the main shoot and the fourth tiller was estimated to be about 10 days. 5) Primordia of GA(3) sensitive internodes were positioned in t he main shoots under the leaf which was newly emerging at the time of GA(3) treatment. Moreover, at the first bract initiation stage, there are two young leaves and two leaf primordia (including b(1)) inside th e newly emerging leaf Hence, it was concluded that because growth subs tances affect the tissues of the sensitive primordial internode betwee n the initiation stages fdr the nag leaf and the first bract:, the num ber of internodes elongated consecutively on the stems in each plant i s either five or six.